How to Plumb Two Aquariums Together

by Jaron Hudson, [email protected], 10/16/2026

A common aquarium plumbing project for the intermediate to advanced aquarist is plumbing two display aquariums together. Why plumb two tanks together? Simply put, aquariums plumbed together share the same water. This means you can keep two display aquariums without testing two different sets of water parameters, or using two heaters, two filters, or two return pumps.

You can even scale this design so that 10 or more aquariums are plumbed together. Because they all share the same system water, maintenance is greatly simplified. Plumbing tanks together is extremely common in fish stores, breeding facilities, and coral grow-out systems because of the simplicity and reduced maintenance.

Reasons to Plumb a Second Tank Into Your System

Here are a few reasons the average hobbyist might want to plumb tanks together:

• Adding a display refugium
• Adding a frag tank
• Adding a seahorse tank
• Adding a predator tank for an octopus, anglerfish, lionfish, etc.
• Adding a different kind of reef tank, like a non-photosynthetic reef or a lower-light reef
• Adding a low-maintenance pico tank
• Adding a dedicated anemone tank

Many of these specialized tanks are super fun, but it can be hard to justify double the maintenance of setting up a second tank. By plumbing a second tank into your current system, you cut out much of that extra maintenance and still get to enjoy the new display.

Step 1: Start with Two Drilled Aquariums

There are many ways to plumb two aquariums together. The simplest and easiest is to plumb both aquariums to one shared sump. To use this method, you will need two drilled aquariums or a hang-on overflow box on each aquarium.

The best solution is to buy drilled aquariums, or to drill both aquariums and install a retrofit overflow box like those from Fiji Cube. If you cannot drill the aquariums, you can use a hang-on overflow box like those from CPR.

Step 2: Make Sure Your Sump Is Large Enough

If you have used an aquarium sump before, you know that when the return pump is turned off, some water drains from the display tank into the sump, raising the water level in the sump. With two tanks, twice as much water will drain into the sump, so the sump needs to be large enough to hold the overflow from both tanks.

If you have one very large aquarium, like a 300-gallon, and want to plumb in a small 10-gallon, the extra water will be negligible. But if you add a second 300-gallon tank to the same system, make sure you have a sump rated for 600 gallons so it can handle the extra water volume during a power outage.

Step 3: Make Sure Your Equipment Can Handle the Extra Water Volume

If you are adding a small display refugium to your main reef system, it won't require any additional filtration. In fact, macroalgae and mangrove trees may actually help clean the water in the main display. But if you are adding a large predator tank or another fish display, you will need to make sure your gear can handle the extra load.

All of the gear in your sump, including filtration, heaters, and reactors, needs to be rated for the new total system water volume. Even with a display refugium or a low-bioload tank, you still need to heat or cool the extra water, so you'll need to upgrade your heater and chiller accordingly.

Step 4: Plan How to Supply Water to the Second Tank

When plumbing multiple tanks together, you need a way to return water to each tank. You can give each tank a dedicated return pump, or use a single return pump to supply all connected tanks. If you are only connecting two tanks, using two pumps may make sense, but with a system of 5 or 10 tanks it is much simpler to use a single pump.

If you use a single return pump, make sure it can supply enough flow for all of the connected display aquariums. You will also need a way to control the flow to each aquarium. Water always takes the path of least resistance, so the outlet closest to the return pump will get most of the flow. Adding a ball valve to each return line lets you partially close it to restrict flow and divert water to the next return line. We recommend In-Line Loc-Line Valves.

Step 5: Connect Your Plumbing

Run the drain line from each connected aquarium into the drain section of your sump. Do not join drain lines unless you dramatically increase the drain pipe size, because combining two drain lines greatly reduces their maximum drain capacity. It is best to run each drain line separately into the sump.

For the return side, you can either place multiple pumps in the return section of the sump, with one pump feeding each display aquarium, or use a single return pump with a tee or splitter to divide the flow between the aquariums. If you use a single return pump, add a valve on each return line to control how much flow goes to each aquarium. The diagram below shows the complete setup.