Aquarium water flow: how much current fish need, and how to increase it

Water flow is how much the water in your tank moves. Fish come from very different places. Hillstream loaches, zebra plecos and many danios live in fast, rocky rivers full of oxygen. Bettas, gouramis and most long-finned fancy fish come from still ponds, ditches and slow, weedy water, or were bred with fins too big to fight a current. Put either group in the wrong water and it slowly wears them down.

Each species card on this site shows a Flow line: Calm, Gentle or Strong. Most community fish are happy with gentle flow. The aim is to match your tank to the fish, and to give every fish somewhere it is comfortable, with no stale corners where waste and low-oxygen water collect.

How to check it

Watch the plants and the surface. Leaves that sway gently and a surface that ripples a little mean gentle flow. Plants bent flat, floating plants spinning round, and a surface with small waves mean strong flow. A glassy, still surface means calm water.
Drop a pinch of food. Watch where it goes. In good flow it drifts around the tank. If it races straight into the filter, flow is strong. If it sits still in one spot, that spot has little movement.
Look for dead spots. A dead spot is a corner or area where water barely moves: behind rocks, under decorations, and in the corner away from the filter. Waste and uneaten food settle there, and algae or a film often grows on the glass.
Work out your turnover. Turnover is how many times the filter moves the whole tank's volume in one hour. Divide the filter's flow rate by the tank's volume. A filter rated 400 litres an hour on a 100-litre (26 US gallon) tank gives a turnover of 4. The number on the box is usually measured with an empty, clean filter, so the real flow in your tank is lower, and it drops as the filter clogs.
Look up each species. Check the Flow line on the species card for every fish you keep.

When it's a problem

Signs flow is too strong:

Fish being pushed around, swimming hard just to stay in place, or swimming head-up into the current.
Fish hiding all day behind the filter, in plants or in corners, or resting on the bottom.
Bettas and long-finned fish with fins clamped to the body, or torn fins (see betta fin rot).
Small fish or fry pinned against the filter intake (see filter intake dangers).

Signs flow is too weak:

Fish hanging at the surface with fast-moving gills, or gulping at the top. This can mean low oxygen; see fish gasping.
Waste and mulm piling up in the same corners, cloudy patches, or a greasy film on the surface.
Fast-water fish (hillstream loaches, zebra plecos) that are pale, hide all the time, eat poorly or don't breed.
Temperature differences between one end of the tank and the other.

Normal: plants move a little, the surface ripples, and each fish spends time where it is comfortable.
Serious: fish gasping at the surface, or fish clearly exhausted by the current. Act today.

What to do right now

If fish are gasping at the surface, add an air stone or lower the water level a little so the filter outlet breaks the surface. Then follow the gasping guide.
If fish are being blown around, turn the filter down if it has a dial, or point the outlet at the glass or along the back wall.
Check the filter is running properly. A sudden drop in flow usually means a clogged sponge or impeller; see filter problems.
Look up every fish's Flow line, and decide whether the tank should be calm, gentle or strong.

What to do next

Fish that want strong flow

Hillstream loaches (such as Sewellia and Beaufortia): live in shallow, fast mountain streams. Seriously Fish suggests a filter turning the tank over 15 to 20 times an hour for Sewellia lineolata, plus extra powerheads and air as needed.
Many plecos, especially Hypancistrus such as the zebra pleco (L046), which comes from flowing, oxygen-rich rivers. Seriously Fish suggests 8 to 12 turnovers an hour, with a powerhead or two at one end of the tank.
Danios, white cloud minnows and many barbs enjoy swimming in a current.
River-dwelling (rheophilic) cichlids, such as some Steatocranus and Teleogramma, which live among rocks in rapids.

Fish that want calm water

Bettas. They are slow swimmers, and long-finned forms tire quickly in a current.
Gouramis, paradise fish and other labyrinth fish, which come from still, weedy water.
Long-finned fancy fish: fancy guppies, veiltail angelfish, long-finned goldfish.
Fry and very small fish, which can be pulled into intakes.

How to increase water flow

Add a powerhead or wavemaker. These are small pumps that sit inside the tank and only move water; they don't filter it. They give a lot of current for little power. A wavemaker turns on and off or changes speed to make a pulsing, more natural current. Aquarium Science suggests aiming one up towards the surface at a slight angle to break the surface, and a second lower down on the other side, aimed at the substrate, so the water circles round and carries waste towards the filter. Size the pump to the tank: a pump made for a 200-litre tank will blast a small one.
Aim the filter outlet. Point it along the long side of the tank, or so the stream breaks the surface. Water that ripples and splashes at the top picks up much more oxygen than water pushed out deep below the surface.
Use a spray bar. A spray bar is a tube with a row of holes, fitted to a canister filter outlet. Placed along the back wall just at or under the surface, it spreads the flow along the whole tank and helps clear dead spots.
Canister vs hang-on-back. A hang-on-back filter hangs on the rim and pours water in from the top. It's easy to fit, but its flow is fairly gentle and mostly goes downwards near the filter. A canister filter sits under the tank and pumps water through hoses. It usually moves more water and lets you put the outlet (or a spray bar) where you want it. For a strong-flow tank, a canister or a large internal filter plus a powerhead is the usual setup.
Add a second filter. Two filters at opposite ends give more turnover, fewer dead spots and a backup if one fails. An extra sponge filter is cheap and gentle.
Add air stones. An air stone driven by an air pump sends up bubbles that move water and stir the surface, which adds oxygen. They add some movement but not much real current. Aquarium Science recommends at least two devices that aerate a heavily stocked tank, so a single pump failing overnight doesn't suffocate the fish. Warm water holds less oxygen, so extra air matters most in summer and in warm tanks such as zebra pleco tanks (see heatwave).
Clean the filter. A clogged filter can lose much of its flow. Rinse sponges in old tank water, not tap water (see cleaning your tank).
Go up slowly. Add flow in steps over a few days and watch your fish. Make sure there are still rocks, wood or caves where they can rest out of the current.

How to reduce water flow

Turn the filter down if it has a flow dial.
Baffle the outlet. A baffle is anything that breaks up the stream: a piece of sponge, a cut plastic bottle, or a mesh bag of filter wool over the outlet.
Put a sponge over the intake or outlet. A pre-filter sponge on the intake also protects fins and fry.
Point a spray bar or outlet at the glass, so the stream hits the wall and spreads out before it reaches the fish.
Use a sponge filter run by an air pump. It gives very gentle flow and is the usual choice for bettas, fry and shrimp.
Add plants and decor in front of the outlet. Tall plants, wood and rocks break up the current. Floating plants calm the surface.

Calm and fast zones in one tank

Most tanks work best with a range. Put the outlet or powerhead at one end to make a fast, well-aerated zone, and place rocks, wood and dense plants at the other end to make calm water. Fast-water fish will sit in the current; calm-water fish rest behind the decor. This works for fish with Gentle flow on their cards and for mixing Gentle with Strong. Fish that need Calm water and fish that need Strong flow rarely belong in the same tank.

Products that help

Powerhead or wavemaker: extra current without more filtering. Choose one with adjustable speed.
Spray bar: spreads a canister's outflow along the tank.
Canister filter: higher flow and a flexible outlet for strong-flow tanks.
Sponge filter and air pump: gentle filtering for calm tanks, fry and shrimp, and a cheap backup.
Air stone and air pump: more oxygen and surface movement. Add a check valve so water can't siphon back into the pump.
Filter with a flow dial: lets you turn the current down for calm-water fish.
Pre-filter sponge: slows the intake and keeps fins and fry safe.
Thermometer at each end of a large tank: shows whether water is mixing.

How to prevent problems

Check the Flow line on the species card before you buy a fish, and keep fish with similar needs together.
Choose your filter for the fish, not just for the tank size.
Clean filters regularly so flow doesn't slowly fade.
Look at the corners each week for piles of mulm and stale water.
Always leave resting places out of the current, even in a fast-water tank.
Have a second source of air or filtering in busy tanks.

Why it matters

Flow does three jobs. It carries oxygen: oxygen enters water at the surface, so moving, broken surface water holds more of it. It carries waste to the filter, so dead spots become dumps for rotting food. And it mixes heat, so the whole tank stays at one temperature. Too much current, though, makes calm-water fish swim non-stop to hold their place. That steady effort causes stress, which lowers their resistance to disease. Fast-water fish have the opposite problem: in still, warm water they may not get enough oxygen.

When to get help

Ask an experienced keeper or your local fish store if fish keep gasping after you've added air and done a water change, if a fast-water species won't eat or settle, or if you're setting up a tank for a demanding species such as hillstream loaches or Hypancistrus plecos and want advice on pumps for your tank size.

In the game

In Multiple Tank Syndrome, water flow comes with the Pleco pack. Each tank has a flow setting of Calm, Gentle or Strong, and each fish card shows the flow it needs. You can change a tank's flow on your turn as often as you like. A fish in flow one step wrong (Calm instead of Gentle, say) gets more tired each turn and is worn out after four turns; a fish two steps wrong (Calm instead of Strong) lasts only two. With the Plants pack, tall plants break the current so calm-water fish don't tire, and floating plants calm the surface for fish that swim at the top. Fish in the quarantine tank never tire from flow. In a real tank, a fish in the wrong current won't die that fast, but it does wear down over weeks.

Sources

Aquarium Science. Turbulence and aeration (surface turbulence, outlet placement, airstones).
Aquarium Science. Wavemakers (pump placement, moving waste to the filter).
Seriously Fish. Hypancistrus zebra (habitat, current and oxygen needs).
Seriously Fish. Sewellia lineolata (hillstream loach habitat and turnover).
Hepper (vet-reviewed). Is my betta's filter current too strong? (signs and fixes).