How to Plumb and Install a Return Line from a Sump to Display Tank

Returning water to the aquarium

While pumping water from the sump up to the display aquarium is fairly straightforward, there are a couple things to consider. Like other parts of plumbing an aquarium sump, there are many ways to accomplish the same thing.

The return line is part of the circulation system — and if you understand what it should do, you’ll realize just how few parts are required.

What are we trying to design here?

  • a securely-mounted outlet that won’t budge or get knocked loose, but we can still disassemble it if needed.
  • minimize the amount of water that flows back into the sump after the return pump is shut off.

How much water do I need to be moving through the system?

This is based on a few factors, and more details are presented here: What is the ideal Flow Rate of Turnover through the Sump?

Choosing Materials

Piping Material

The most common choice for plumbing is PVC – a time tested material that is both cheap and easy to find at hardware stores in various sizes. This guide is based mostly around the use of PVC.

Info about how to choose pipe material is presented on Plumbing a Reef Tank – What Pipe Material Should I use?

What size of Plumbing line and fittings should I use?

As for pipe diameter, your pump will dictate what you should go with, but it’s a good idea to go up a size. This will also allow the pump to not lose any head pressure from constricted pipe diameter. Many submersible pumps will have a 1/2″ outlet, and as a general rule most people don’t use anything smaller than 3/4″ plumbing for a return line.

It’s a good idea to just go ahead and use 1″ pipe for your return. It’s not much bigger in appearance, and you won’t lose any flow due to friction loss of a smaller diameter pipe.

Connecting the Pump to the Return Line

PVC union fittingReturn pumps need to be removed and cleaned out, usually every few months. If you are running a high-calcium stony-coral-type tank, then calcium deposits will form on the pump over time. This can eventually lead to it seizing up completely. As part of regular maintenance, most people take the pump out, remove the impeller, and soak it in a vinegar solution for a few hours. It can then be scrubbed with pipe cleaners or a stiff brush, and put back in.
For this reason, you will want a union just above the pump to be able to disconnect the line.

Dampening Vibrations from the Pump

Return pumps are also known to produce annoying vibration in your sump, and if you are running a large model, it can be quite loud. A silicone “Trivet” made for Hot cooking pots is a good addition under the pump. These are available from any store that sells kitchen supplies.

There is more info here: Selecting the Proper Return Pump for your Reef Tank

How the Overflow is Affected

Can I Tee off the return to divert some water elsewhere?

Some people will use a “T” fitting to direct a portion of the flow to a valve or series of valves. These extra outlets could do things like feed a reactor, refugium or skimmer.

If you are using a siphon based overflow method like a Herbie or Bean Animal drain, then the balanced flow of the siphon will be affected. Even small variations in flow will throw the balance off and it will need to be adjusted more frequently than is convenient.

Mounting the Outlet to a Tank with Trim at the Top

Since this is where all the water from your tank passes through, it’s best not to cut corners with how you mount the return outlet to your tank and make sure it’s completely secure. Ideally you want it permanent, with the ability to disassemble it if needed. You want to make sure that the line can’t be knocked loose, which could result in a flood.

  1. Street 90 elbow fittings allow the pipe to point 180 degrees in a tight curve without extra pieces of PVC. street 90 fitting
    The parts that form the U shape should hang on the display tank and should fit snugly so it doesn’t move around.
    In order to get a custom fit, you can play around with how far the pieces insert to one another, or even cut pieces to make them more compact. Make sure to glue the fittings when you’re done.
  2. At the very least, you should mount the return line to the stand somehow using a PVC mounting bracket that screws in.PVC conduit clamp for securing a return line These are found in the electrical conduit section of hardware stores.

return line reef tank eurobrace

Should I install multiple return lines?

This is generally not necessary, and I will show you why.

If you inject a visible additive like an Alkalinity solution down at the intake of the return pump, you can observe just how quickly water mixes in a tank.

With adequate powerhead flow, all that water will mix together into a uniform haze in a few seconds with even low amounts of flow. This happens whether the the return is one outlet or many different ones. This is a good indication of why you need not worry about multiple outlets or placing an outlet as far away from the overflow as possible. Consider the following:

  • Splitting the return can unnecessarily add to friction loss of the return system, lessening the output of the pump. In order to compensate for this, you would need multiple outlets – which are unsightly and provide no benefits
  • Unless you have a 300+ gallon system or an 8′ long tank, there is no need to use more than a single return outlet
  • A single return line is inconspicuous and easy to hide out of sight

Back-Siphoning through the return line when power to the return pump stops

When the return pump stops, the filled-with-water return line will immediately reverse the direction of it’s flow via siphon action and start sucking in water from the main display. This is usually referred to as “Back Siphoning” because it is unintended.

The amount of water that drains during these few seconds can be minimized but not eliminated. The sump should have ample capacity to handle it.

Ways to avoid excess Back Siphoning

Install the return outlet nozzle just beneath the water surface

Simply by designing your outlet’s opening to be underwater by about 1/2″, you can eliminate excess back flow into the sump without the need for a check valve or any special fittings or methods. Other than sufficient sump capacity, this is all you need. This is the method explained in the diagram above.

Just for the sake of education, I will include the following info about some common mechanisms added to return lines. I don’t recommend either because of their likely probability of failure.

Other Methods – Click to Expand

Install a Check Valve

pvc check valveSome people opt to install a check valve somewhere in a vertical run of the line above the pump to limit back-siphoning. While very common, they are not necessary in every tank.

There are many types, some better than others. They all have the same shortfall – Stuff grows in them. Coco worms, snails, pieces of algae, anything. Once the seal is obstructed, they are useless.

You may enjoy months of trouble-free operation before this happens, but it will.

It is better to alter your sump design and minimize back siphoned water with the correct height of the drain inlets and return pump outlet. This is quite simple to do.

It goes back to the “Know thy self” philosophy.

“I’ll just clean it out” – yeah sure. The amount of people that clean the inside of their plumbing is about the same amount of people that clean the underside of their car.

Drilling a small “Anti-Siphon Hole”

– A small hole or two can be made in the pipe just under the surface of the water. As soon as this hole sucks in air, the siphon will be broken. These holes are usually made around 1/8″ in diameter — as anything smaller might not be able to reliably break the siphon. They are usually placed about 1/2″ beneath the surface of the water to avoid turbulence or create micro-bubbles.

Watch out – you should be aware that a small hole is very susceptible to getting clogged by debris or some kind of livestock in the tank. During normal operation at a positive pressure (water going out) this is unlikely to happen, however when flow stops, water will start to suck in through the hole, and it can tend to slurp up any surrounding algae or debris. It is also possible for the small hole to become completely covered by the growth of Coralline Algae. If the anti-siphon hole is clogged, the siphon continues until some air eventually enters the line from the main nozzle. If this is normally several inches beneath the surface, this can be a lot of water!

Have a backup plan – like sufficient sump capacity to account for this possibility.

The Spray factor – Under pressure, a small hole will also tend to blast out water, similar to a garden hose that springs a leak. This mini-jet of water can create noise, excess bubbles or put water where you don’t want it. For these reasons you might just want to avoid using an anti-siphon hole because of these potential problems.


  1. very helpful thank you,have you any idea what size hole would I need for a spilt sump ie to smaller tanks joined together by a pipe should I need two small ones or one big one ? thanks

  2. Just leave them little fish and corals in the sea and the plumbing issue becomes a no brainier!

  3. Yes, I’m just being a silly “dummy”! Love the website and have learned much from it! Leave it alone!

  4. For the return piping how big to I want the pvc pipe that is going up, and do I need the same size for the streets? I do not plan to have a union.

    • Pvc not pac

    • It depends on the tank, but in most cases 1″ would be the minimum. In order to maximize the flow you get from your pump, you don’t want to go down in diameter from the pump’s connection. It’s a good rule of thumb to go up by 1/2″ with a bushing that connects into the main return line.

      The street 90’s would be the same size as the main line. Also – I would recommend at least having one union just above the pump, extended by enough to keep it out of the water. You are going to need to clean the pump eventually.

    • So for example if I was to get a 3/4″ (Pump size) 2 foot long pvc pipe do I want to get a 3/4″ street? If that is correct wouldn’t the pipe be to big to fit?

      • In this example, yes. The street elbows are flared on one end to fit over existing 3/4″ pipe. The other end of the elbow is standard 3/4″ size which can go into another street elbow or connect to 3/4″ pipe with a coupling.

  5. Mark Gallo says:

    I’m designing my system and plan on using 1″ pipe but loc-line doesn’t come in 1″ size. Suggestions?

    • I am less of a fan of loc-line now as I mostly think it’s unnecessary. Many people feel the need to use a 6″ or so length of it to “direct flow” but open themselves to risk. A return outlet using a length of loc-line can be bumped or moved unintentionally — which means your return outlet is pushed further under the surface. This could lead to more water back-siphoning from the display tank and overfilling the sump. I would leave it out of your design if possible.

      • Randy Helms says:

        GMAC I am planning on placing loc-line in my new tank, and purchased a Y splitter for it. I was planning on setting one really high close to the surface behind my aquascape, and one lower in the water column. When I lose power and it starts back siphoning, the first line to draw air would kill the siphon correct? I am pretty sure the answer is yes, but just making sure. I plan on installing a check valve as an added measure near the manifold.

        • Yes, as soon as air was drawn in from one of the inlets, the siphon would stop. I don’t like using long lengths of loc-line because of the back-siphoning issue. Also – splitting the return into many outlets is good in theory – but often only produces only a little extra flow in the tank.

  6. Paul Daniel says:

    I am venturing into my first sump and needed some advice on the return line pump. I plan to use a danner mag 18 which has a 3/4 mpt outlet with an inside diameter 1 1/2 based on my research. I want to use one inch PVC pipe for the return line into Display tank. What connections should I use to connect the pump to the one inch PVC pipe?

    • Here is how I do it, top to bottom:

      1″ PVC to tank / return fittings
      1″ union
      1″ PVC (long enough to get union out of the water)
      3/4″ fpt to 1″ slip reducer bushing
      pump with 3/4″ mpt fitting

      • Paul Daniel says:

        Thank you so very much. That helps me.

      • Eric Beavers says:

        What is the significance of keeping the union out of the water?? Is there an issue with it being submerged?

        Thank You!

        • No issue with it being submerged, but having it out of the water keeps things from growing on it and from it getting crusted up which can make it harder to loosen after several months. Less things to clean.

      • Paul Daniel says:

        Seems I made a foul, bleep and blunder…I finally got around to the pump and having the sump build…Now to where I think I messed up the mag 18 seems to be powerful for my setup as the water is leaving the section with pump quicker that it is being replaced. Any suggestions on how to correct. I guess options are ball valve and reduce flow or use a 3/4 pvc pipe or the last resort get a less powerful pump. That is me just thinking out loud but with your experience you may have a better solution.

        • You essentially have 2 options when plumbing for an oversized pump that is pushing too much water.

          1. Use a valve to reduce the flow
          2. Use a combination of a Tee and a valve to plumb it so you divert some flow back into the sump.

          – I prefer the 2nd option because that way you can at least add some circulation to your sump.

          Getting a new pump might ultimately be the best option, as changing the plumbing won’t help with noise. You may be able to offer a trade with someone in a local reef club or Facebook group to get something a bit smaller and better suited. Also – it’s never a bad idea to have a backup return pump on hand.

          • Paul Daniel says:

            Thank you for your reply. Ok will probably look to replace the pump later this year. Will look at using the Tee with the valve. Any recommendations about where in the plumbing to install both. I am assuming the Tee will be just over the return pump area

          • Paul Daniel says:

            Ok I went back and gave this some thought as I know I could not have made that big a mistake with the pump as I researched this thoroughly before I made the purchase. It was a simple issue of inflow v outflow – The standpipe in my overflow area was not draining to full capacity as the water level was too low. Once I ensured the water level was correct it started to work like a dream. Thank you for your help as this is my first time using a sump, doing the plumbing etc and your assistance aided me greatly.

  7. Paul Martin says:

    I have a M1 Vectra return pump by eco tech marine. The outlet of the M1 pump is 1.25 inches pvc. I want to use this pump for my return and to feed my ATS. I want one pump to minimize noise in my family room and also my sump has limited space. It is a 50 gallon DT with with a ~15 gallon sump, herbie set up. The return connection to my DT is 3/4 inch. I am using vinyl tubing for the return to the DT. How would you plumb the M1? I am trying to minimize the restriction to flow and I want to vary the flow to the ATS; so I thought I would install a valve after the take off to the ATS (on the return line to the DT). That way If I restrict flow to the DT I would divert more flow to the ATS. I will install a union. Should the union be above the water when the pump is off or when it is on? When I transition from 1.25″ to 3/4″ is it better to make that transition close to the pump or higher after the union, (after the take off to the ATS)?
    I greatly appreciate this awesome website and used it for my build. Thank you! Paul

    • Hi Paul – I would probably re-plumb the return line if possible with 1″ flex PVC. Vinyl tubing has the most drawbacks and potential for failure in reef tank plumbing. This post goes over pipe material:

      The specs on the M1 pump seem a little oversized for your 50 gal application, but I realize it is a DC controllable model. From what I have read the British threads are hard to find fittings for to convert it over to US standard size. I am not sure why they went with a barbed fitting on the pump – almost all others have a threaded fitting.

      In your case I would try to use a short length of silicone tubing coming from the pump. This short (under a foot) piece would then attach to to a barbed fitting that inserts into a slip x threaded coupling. from the coupling you can plumb to flex PVC up to the return outlet.

      Using a short length of tubing to reduce the transfer of vibrations is a common technique. Doing this also safegaurds your tank against the pitfalls of barbed fittings – a pressure blowout where the fittings come apart. If you only use a short length of tubing – a leak should be contained in the sump. There is more info about this in the post I linked to above.

      As for your other q’s – a union is usually best installed above the waterline (pump on or off) just for ease of adjustment. I will be updating my diagrams. Also I don’t think the location of a reducing bushing or other reducer makes much difference.

      Glad you like the site – thanks for commenting.

  8. Chris McKillip says:

    I am starting to plumb my Sump, I have two bulkheads, supply and return. I have a Durso for the supply that I am plumbing a ball valve past the bulkhead, couple of 45’s to let the pvc not run horizzontal down to the sump. My pump has a 1/2″ fitting up to a 3/4″ ball valve, Backflow then up to a 90 deg leg – I will be sizing the pipe at this point with 1″ PVC running back to the bulkhead. Anything I need to look out for? I am totally new to the sump design. Sump will have a ATO, and a in Sump skimmer.
    120 gallon reef.

    ALSO – Is there a difference in normal plumbing supplies (HOME DEPOT) vs Reef Tank

    • gmacreef says:

      A pic would help to identify any potential problems – it’s hard to explain this stuff in writing. As for plumbing supplies the PVC you get at Home Depot is the stuff you use on your reef tank. Check out this post I wrote about types of plumbing.

  9. Dear GMAC REEF,
    great website, really helpfull .

    I am wanting to redo the plumbing on my 120 gal reef which has been running for a few years now.. The reason is the original plumbing is done with a 3/4 inch pvc which I now want to change to 1 inch for more flow through the sump..
    I am using Fluval sp4 which is rated for 1800+ gallons so I am hoping 1 inch return will be more suitable ??
    Also should the return and the drain both be of same size or I need to up the size of return ? I am also running 2 reactors via return pump.
    Lastly, as I plan to redo the plumbing on existing reef how long should I let it cure before running the water through it ?

    • gmacreef says:

      The return line and drain don’t have to be sized the same. One is driven with a pump and the other is just using gravity so their sizes are not related in the system. 1″ pipe can move a lot more water than 3/4″ can. With PVC cement, you don’t have to wait more than a few minutes before running water through it, but try to do a rinse with freshwater if possible.

  10. Starting a new project – I inherited a 125gal – 72″ long tank from a family member, who never had it up and running – so lots of fun ahead. the tank is manufactured with two rear overflows, each with two holes in the bottom – I plan to follow your advice on setting those up wit Herbie overflows, as its in a noise sensitive area, probably just using one and abandoning the other.

    For return flow, I assume, there are two holes in the floor of the main tank, against the back wall, and just to the inside of each overflow column. I was considering either abandoning one, and running a stand pipe neat the top, with some adjustable discharge blades, or running two standpipes with discharge outlets, and maybe tee them together with a SCWD. I’m afraid the SCWD might mess with the Herbie set up too much.

    Any suggestions on a dual return line arrangement? nice thing about a 72″ long stand, is the amount of room to work with underneath.

    Appreciate any help, and thanks for all this valuable information in one place.

    • Personally I think I would cap off the two existing holes inside the tank (the ones not inside the overflows). Then I would run a single return line over the tank trim to position the outlet up high and pointing towards the middle of the tank. I wouldn’t bother with any SCWD or other nozzle contraptions. Having a single return line seems to maximize what your return pump can put out, giving the best bang for your buck as far as flow is concerned. It is also very simple. The only time I would really consider dual returns being necessary is if the length of the tank was 8ft or longer.

  11. Just to clarify, there are two holes in the bottom of the tank, along the rear wall, and centered between the 2 overflow columns. The 2 overflow columns are located, at the back LH and RH corners. Also, the tank did come with a couple of perforated stand pipes, each with multi-segment foam sleeves, not sure how this would be applied to either intake or return piping. Again, thanks for any advice.

  12. Neil Natic says:

    I have a tank that does not have overflow or return holes in it and I plan to cut them. Your diagram shows the return going over the lip and into the tank. Is this better than having a third hole (one overflow and one emergency overflow)?

    • I personally like having the return go over the top trim of the tank because it allows you to position the outlet high up. This limit’s back-siphoning without the use of a check valve. This is mostly preference but I wouldn’t say it’s better. If you are creating your own overflow then I would have 3 drains and use a Bean Animal style setup, while running the return separately.

  13. I’m setting up a new 90 gal tank and have a question as to ideal flow direction through the sump. Which is preferable: to have the shortest piping from return pump to tank or from overflow to sump? Specifically, in my case the overflow and return plumbing is in the right rear corner of the tank. Would it be better to have the sump intake drain or the return pump right under the overflow? I’m trying to maximize overall flow (and efficiency as the pump is DC), but there may be other considerations I’m not aware of. Also, the tank comes with flexible tubing and threaded barb inserts to thread into the return and overflow drain; should I use this or use threaded pipe? Thanks.

    • A few more feet of lateral distance won’t add any measurable head pressure to the return pump, so flow from the outlet isn’t really affected by that.

      Adding horizontal distance the pipes have to travel can negatively affect the drain setup. A siphon has a harder time purging air downwards if there is too much horizontal travel in the line. When designing your sump and system, sometimes it’s hard to avoid having a bit of horizontal run in your drain lines. If you can limit the distance then the siphon will start quicker.

      For the fittings I would recommend, there is more info on this article Materials for Plumbing Reef Tanks. In short, If you have the chance to upgrade to PVC fittings, go ahead and do that if you want to maximize both return outlet flow and drain capacity.

  14. Mel Easton says:

    This is great help. Thanks. Can I have a little advice as to why I would need a stop tap on my pipe work from my marine tank to the sump please other than for water changes?

    • The valve is used for creating a siphon – where you close off the flow a little bit. If you match the amount of water draining the tank to the amount being returned to it, you can create a quiet drain with the help of a siphon. The Herbie drain and BeanAnimal design both use this concept. Thanks for commenting.

  15. Jim Dutton says:

    I’m working on the pump return pluming for my 75g tank from a 20L sump. I purchased a Jebao DCT 6000, and the pump outlet is 1.25″ MPT. I planned on adapting this to a 1″ eshopps Flexihose to get me back up to the top of the aquarium. Now I’m looking for a 1″ return nozzle that either screws or glues to 1″ PVC…and am shocked to find nothing out there? Could I be missing something? I purposely upsized the pump to account for the head difference between the floor of the stand (where the pump is mounted) and the top edge of the tank, as well as the potential for upsizing the tank later on. I don’t want to restrict the flow at the end of the pipe with a 3/4″ nozzle if I can help it.


  16. Mike Conigliaro says:

    return line from the sump at the tank???
    1). straight down
    2). 45 degree elbow
    3). 90 degree elbow
    return pump is a Sicce Syncra SDC 9.0

    • It doesn’t matter. On the return line, the fittings chosen makes such a small difference in the resulting pressure that it is hard to measure. BRS did a video on this:

      • Mike Conigliaro says:

        my question is more of do I want the flow going down or do I want the flow across the surface?

        • Ahh – well in my case I have it pointing about 30deg down. I like having the return plumbed up as high as possible in the tank, to minimize back-siphoning. So I place it under the waterline just enough to prevent air being drawn in from a vortex, and giving some surface agitation.

  17. Ted Reynolds says:

    I have a 7′ long 420 gallon tank that I’m setting up. The return pump is a reeflo snapper/dart gold hybrid with a 1.5″ outlet. I split that in to three 3/4″ return lines (one on each end and one in the middle) ran to 3/4″ bulkheads with loc-line inside. Was this not the way to go? Should I have ran 1.5″ all the way up, then reducing to 1″ bulkheads on the returns? Thanks.

    • I wouldn’t say what you did was bad. The location of where the pipes are reduced on the drain line doesn’t generally make much difference. For the most part I think that 2 returns are all that’s needed and that is only if you have a very long tank such as yours.

  18. First of all, thanks for this excellent site. It did make me realize how dumb I have been with my plumbing. Now here is my dilemma. I’ve stepped down from a 300 gal to a 90 due to a blowout of the 300, long story on that one. Anyway due to a DIY stand, the DT sits much higher than a bought stand. This of course effects the head pressure of the pump and return flow to the DT. The DT has an overflow with 1″ return and 3/4 feed trough it. currently I have it plumbed as follow: Mag1800 pump >check valve > union > tubing > gate valve > pipe trough bulkhead > lock line > step down to 1/2″ lock line > flare nozzle to DT. I now realize the step down limit the pump output even more than I intended.

    Since I do have a second mag1800 from my previous tank, I was thinking of adding it to the system to increase the flow. From my research on this I deducted the following set up:
    Pump 1 > check valve > union> tubing > one side of wye (as I understand it, use of check valves on both pumps are imperative for dual pumps to one output)
    Pump 2 > check valve > union> tubing > second side of wye
    Wye > gate valve > pipe> 3/4 lock line, round nozzle to DT. ( removed the step down mistake from previous set up)

    This should give me plenty return to DT. What are your taught on my planned set up?

    • If you have a Mag 18, then that pump is plenty capable of running your 90 gal on its own. It would be a good idea to redo the fittings going into the tank so the plumbing size isn’t reduced quite so much. At the very least upsize the 1/2″ section of Loc-line.
      There doesn’t seem to be anything wrong with your proposed 2 pump setup, but again it seems like too much flow. I would also say that 1″ diameter pipe all the way to the tank would be necessary in that case.

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