How to Set Up a River Manifold (One-Way Flow) Tank

Most aquarium filters push water around in circles: out of the outlet, around the tank, back into the intake. A river manifold tank does something different. It sends water in one direction, from one end of the tank to the other, in a smooth sheet, like a stream running over rocks. Fish that come from fast water love it, and it looks amazing.

How a manifold works

A manifold is a pipe with several outlets (holes or nozzles) running along one end of the tank, usually across the width near the bottom and mid-water. Water from the pump or filter comes out of all those outlets at once, which spreads the flow across the whole end of the tank instead of one strong jet.

At the other end, the water is pulled back out through an intake, an overflow, or a sump. The result is unidirectional flow: water moving steadily from one end to the other.

What you need

• A long tank. Longer tanks give the current more room to run. A 4-foot tank is a popular size, but smaller works.
• A pump or canister filter with enough flow. Hillstream setups often turn the tank volume over many times an hour.
• PVC pipe and fittings to build the manifold (aquarium-safe; plain, uncoated pipe).
• An intake at the far end with a sponge prefilter to protect small fish and shrimp.
• Smooth river stones, gravel or sand. See best substrate for a river tank.
• A tight lid. Fast-water fish can jump.

A simple DIY layout

1. Plan the flow direction. Manifold on one end, intake on the other.
2. Build the manifold: a length of pipe that fits across the tank's width, capped at both ends, with a row of holes drilled along one side. Start with fewer, smaller holes; you can always add more.
3. Connect it to the outlet of your pump or canister.
4. Mount it low along the end glass, holes facing down the tank.
5. Put the intake at the opposite end, with a sponge on it.
6. Arrange rocks to create fast runs, slower pools and sheltered spots behind larger stones.
7. Turn it on and watch. Adjust holes or add a valve if the flow is too strong.

Some hobbyists use a sump with an overflow so the water level stays steady; others run a powerful canister. Both work.

Get the flow right

• Not every spot should be fast. Fish need resting places. Put larger rocks and driftwood in the current to make pockets of slower water.
• Watch the fish. If they're pinned against the glass or hiding constantly, it's too strong.
• High oxygen. Fast water breaks the surface and adds oxygen, which most stream fish need.

Fish and critters that love it

• Hillstream loaches
• White Cloud Mountain minnows
• Danios
• Stream gobies
• Bamboo shrimp, which filter food from the current

Keep it clean

Strong flow keeps debris moving toward the intake, which makes cleaning easier. Rinse the prefilter sponge often in old tank water, and keep up with water changes.

Cycle first

Like any new tank, a manifold tank needs to be cycled before you add fish. See how to cycle a new aquarium.

A one-way-flow tank takes a little more planning than a standard setup, but watching fish hold their place in a real current is worth it.

Related: River Tank EcoSystem Owners Guide