Carries debris to the filter
Good flow keeps waste moving until the filter catches it, instead of settling in corners.
Pumps & flow
Pumps and water flow
Moving water carries waste to the filter, spreads heat, and brings oxygen in at the surface. How much you need depends on what lives in the tank: a betta wants a gentle current, while many reef corals need strong, changing flow.
What it does
Why water movement matters
Flow is part of life support. Still water lets waste settle, oxygen drop, and heat build up in pockets.
Moves the surface
Oxygen enters at the surface. A steady ripple across the top lets gases trade in and out.
Keeps the tank even
Flow spreads heater warmth, top-off water, and additives so the whole tank stays the same.
Feeds and cleans corals
Corals rely on moving water to bring food, carry waste away, and keep sediment off their tissue.
Pump types
Return pumps vs powerheads
A return pump runs the filtration loop. Powerheads and wavemakers move water around inside the display.
Sump back to the tank
Pushes filtered water from the sump up to the display. Size it to the overflow and to the height it has to pump.
Shop pumpsCirculation in the display
A submersible pump that moves water around rock, sand, and livestock. It does not filter anything.
Shop wave makersFlow that changes
A controllable pump, or a controller for pumps, that varies speed and pulses. Changing flow reaches more surfaces than one steady stream.
Filter output and air
In most freshwater tanks the filter return provides the flow. A spray bar or air-driven sponge filter softens it for gentle fish.
Shop air equipment
How much flow
Turnover is a rough guide
Turnover is how many times per hour your pumps move the tank's volume. Rockwork, placement, and livestock change what the water actually does, so treat these as starting points.
Gentle, low flow
Bettas, fancy goldfish, and other long-finned fish tire in strong current. Use a gentle filter, soften the output, and leave calm resting areas.
Filter around 4-6x per hour
A filter rated around 4-6 times the tank volume suits most freshwater tanks. Plants should sway gently, not get flattened.
Often around 10-20x total
A powerhead or two plus the filter or return usually gives marine fish steady circulation and good oxygen.
Often 20-50x total
Mostly from powerheads. Many soft corals and LPS prefer the gentler end and many SPS the stronger end; watch how each coral responds.
Often 3-10x through the sump
The return only runs the filtration loop. Match it to what your overflow and skimmer can handle at your real head height.
Setup tips
Get the flow you paid for
Where a pump sits, and what it pushes against, matter as much as the number on the box.
Read the flow chart
Return pump ratings are usually at zero height. Lift, elbows, valves, and long runs all cut real flow.
- Measure from sump water level to the outlet
- Read the pump's chart at that height
- Use fewer elbows and the right pipe size
Find the still corners
Watch where food and debris settle. Those spots need more movement.
- Aim pumps across the tank, not at the sand
- Leave gaps behind and under rockwork
- Two pumps often beat one big pump
Ripple, do not splash
Aim a return or powerhead to keep the surface moving. Add an air stone in freshwater if fish gasp.
- Warm water holds less oxygen
- Heavy splashing drives off injected CO2
- Surface film is often a sign of weak movement
Plan for power outages
When a return pump stops, water can siphon back down the return line into the sump.
- Keep the outlet near the surface or add a siphon-break hole
- Leave room in the sump for that backflow
- Guard intakes for small fish and shrimp
Maintenance
Keep pumps running strong
Pumps slow down gradually, so a drop in flow is easy to miss until the tank shows it.
Look and listen
Check that every pump runs, the surface moves, and intakes are not clogged with debris or algae.
Clean the impeller
Unplug, remove the impeller, and clean the housing. If the manufacturer allows it, soak off mineral buildup in diluted vinegar or citric acid, then rinse well.
Notice slow drops
If corals stop swaying or debris starts settling, a pump may be clogged or worn. Clean it before buying another.
Rattles need attention
Grinding, rattling, or new vibration often means a worn impeller, a loose mount, or debris inside.
Common mistakes
What usually goes wrong
Most flow problems come from too much in one place and too little everywhere else.
Aiming a pump right at coral
Tissue can recede or polyps stay closed. Aim flow past corals so it moves them gently.
Coral not openingNo surface movement
Fish may gasp at the top, especially in warm water. Add ripple or an air stone.
Fish gasping guideReturn stronger than the drain
If the pump outruns the overflow, the display can flood. Size the return to the overflow, or turn it down.