How to plumb and install a Return Line from a Sump to the 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.

  • david

    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

  • mightymo

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

    • Maybe I should delete this whole website

      • Donfenn

        Noo this site has been more than helpful. Mightymo is just an ignorant dummy that’s trolling. Ignore.

        • I wasn’t serious and I don’t think he was either.

        • mightymo

          Trolling is a great fishing technique.

  • mightymo

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

  • Matt

    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.

    • Matt

      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.

    • Matt

      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.

  • Mark Gallo

    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.

  • Paul Daniel

    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

        Thank you so very much. That helps me.

  • Paul Martin

    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: http://gmacreef.com/reef-tank-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.