Key takeaways
- Size in-tank pumps on the longest straight run in the tank, budgeting one pump per 60 to 70 cm of length rather than one large unit.
- Assume any pump delivers 25 to 40 percent less than its rated flow once media, hoses, and lift are in place.
- Turnover targets range from 4 to 6 times volume per hour on a freshwater community tank up to 30 to 50 times on an SPS reef.
- Return pumps are chosen on the manufacturer head curve at your actual lift, not on tank volume.
- Never throttle a submersible pump outlet to cut flow, because the extra bearing load shortens shaft life.
The 1450 litres per hour printed on a Fluval 407 box is measured with an empty basket, no hose, and no lift. Fill the trays with Seachem Matrix, run two metres of tubing up to a spray bar, and the real figure lands closer to 900. That gap is where most flow problems begin.
Sizing water movement is really three calculations stacked together. How much total turnover the tank needs, how far the water has to travel to get there, and which class of pump produces the pattern you want. Get one of those wrong and you end up with a tank that looks fine and a corner packed with mulm.
Turnover is a starting point, not an answer
Turnover means tank volume moved per hour. A 200 litre tank fed by a 1000 litre per hour filter is running 5x turnover. Useful shorthand, poor specification.
The problem is that turnover treats the tank as a bucket. Two 200 litre tanks, one 120 cm long and 45 cm tall, the other a 60 cm cube standing 55 cm deep, need completely different equipment to hit the same nominal number. The long tank needs reach. The cube needs vertical circulation.
Use turnover to pick a rough class of pump, then test that choice against the tank’s actual dimensions. Because target turnover shifts with stocking and livestock, treat the bands below as starting points rather than rules.
| Tank type | Total turnover | Notes |
|---|---|---|
| Freshwater community | 4-6x | Filter alone usually covers it |
| Heavily planted | 5-8x | Spread wide and slow, not jetted |
| African cichlid | 8-12x | High waste load, needs sweeping |
| Goldfish | 8-10x | Heavy solids, frequent mechanical cleaning |
| Shrimp and fry | 3-5x | Sponge intakes or baffled outputs |
| Soft coral reef | 15-25x | Mixed direction, variable |
| LPS reef | 20-30x | Indirect, no direct blasting |
| SPS reef | 30-50x | Gyre or controllable pumps |
Why dimensions beat volume
Water leaving a nozzle slows fast. A 2000 litre per hour circulation pump with a wide outlet will push a current you can actually see for maybe 60 to 70 cm before it breaks down into general drift. Past that point, the far end of the tank runs on whatever return flow happens to arrive.
Measure the longest straight run in your tank and ask honestly whether one pump can cover it. On a 120 cm tank, a single powerhead at one end almost never does. Two smaller pumps facing along the same wall, or one at each end offset slightly to form a loop, beat one large unit every time.
Depth counts just as much. Anything past roughly 50 cm tends to stratify unless something drives water down the back glass and along the substrate, which is the point at which the exact height and angle of the pump stops being cosmetic.
In-tank pumps are rated at zero head with clean impellers. Canister and sump pumps are rated the same way, then lose output to media, lift, and every elbow in the plumbing. Assume 25 to 40 percent less than the box claims once the system is built.
Powerheads, circulation pumps, and wavemakers
These three names get used interchangeably in shops and they describe different tools. Buying the wrong one is the most common sizing mistake I see.
A powerhead is a small submersible with a narrow outlet and an impeller. Designs like the Hydor Koralia Nano series move a modest volume in a fairly tight cone, which suits driving an undergravel plate, feeding a reactor, or breaking surface film on a nano tank.
A circulation pump uses a propeller behind a wide shroud instead. Something like an Aqueon Circulation Pump 1250 moves several thousand litres per hour, but slowly and across a broad face. That is the correct tool for general movement in a display tank.
A wavemaker is a circulation pump with a controller that varies output on a cycle, giving pulsing, alternating, or pseudo random flow rather than a constant push. Whether the controller earns its price on a freshwater tank depends mostly on whether your plants flag in steady current.
| Type | Typical output | Pattern | Best for |
|---|---|---|---|
| Powerhead | 400-1500 lph | Narrow cone | Nano tanks, reactors, targeted lift |
| Circulation pump | 1500-8000 lph | Wide, low velocity | General movement in tanks over 90 cm |
| Wavemaker | 2000-15000 lph | Variable, pulsed | Reef tanks, large planted displays |
| Return pump | 600-5000 lph | Plumbed, single direction | Sump systems only |
Return pumps are sized on a different number
If you run a sump, the return pump is not chosen on turnover. It is chosen on head height: the vertical lift from pump to outlet, plus friction losses through every metre of pipe and every elbow.
A Sicce Syncra 1.0 rated at roughly 950 litres per hour on the bench will deliver visibly less at 1.5 metres of lift, and less again once the plumbing turns three corners. Manufacturers publish a head curve for exactly this reason, and reading that curve properly separates a quiet sump from a gurgling one. Sicce and similar makers list the curves in their product documentation.
Target 5 to 10 times display volume through the sump and stop there. Pushing 20x does not clean anything faster, it just cuts contact time in the sump and makes the overflow noisy.
A worked example on a 240 litre planted tank
Tank: 120 x 45 x 45 cm, around 240 litres, heavily planted, pressurised CO2, thirty small fish. Filter: a Fluval 407 rated 1450 litres per hour, realistically delivering somewhere near 950 with full media and a spray bar.
That is roughly 4x turnover, which is plenty for the biological load, and it still left the far left corner dead. The spray bar pushed water right to left along the back glass, but the current had given up by the 90 cm mark. Fine plant debris piled up there, and the drop checker on the left read blue while the right one read green.
The answer was not a bigger filter. One small circulation pump of about 1500 litres per hour on the left end wall, aimed low along the front glass and back toward the filter intake, closed the loop. Both drop checkers matched inside three days.
Total in-tank movement came to around 2450 litres per hour, near enough 10x. That sounds aggressive for a planted tank and it is not, because the pattern is a slow circuit rather than a jet.
The symptoms of too much flow
Fish report it first. Long finned bettas, fancy goldfish, and most gouramis will hold station behind hardscape or hug the substrate instead of using open water.
Plants report it second. Stem plants leaning permanently downstream is normal. Fine leaved species such as Rotala shedding lower growth, or leaves physically stripping, is not.
Substrate reports it third. Sand scouring into ripples or bare patches means the outlet is too close or too tight. Widen the nozzle or lift the pump before you reach for the speed dial.
Never fix excess flow by throttling a submersible pump’s outlet. Restricting the discharge on a small propeller pump raises load on the bearing and shortens shaft life. Reduce speed electronically, redirect the flow, or fit a smaller pump.
When the flow you paid for quietly disappears
A pump running for eighteen months rarely produces what it did new. Ceramic shafts wear a groove, impeller magnets pick up a calcium crust, and the volute clogs with fine silt.
Losing 20 to 30 percent of rated output over two years is ordinary in hard water. Catching that wear before the shaft snaps is a ten minute job on a schedule and a livestock emergency otherwise.
Check the boring causes too. A kinked hose, an intake pressed flat against the glass, or an air lock trapped in the return line explains a large share of flow complaints, and none of them need a new pump.
Air pumps follow a different rule
Air pumps do not move water directly. They move gas, which lifts a column of water behind it, and their real limit is pressure rather than volume.
A pump that is comfortable on a 45 cm deep tank can stall on a 60 cm one with identical litres, because the water column pushing back on the diaphragm scales with depth alone. Size those on depth first and tank volume second.
Buying order that avoids rework
- Measure length, width, and depth in centimetres before opening any product page
- Pick a turnover band from the table above, then check it against the longest run
- Budget one in-tank pump per 60 to 70 cm of tank length
- Read the head curve, not the headline figure, for anything outside the tank
- Leave 20 percent headroom for impeller wear and media restriction
Manufacturers vary in how honestly they publish these figures. Established brands such as Eheim tend to state flow with media in place, which makes their numbers look worse and behave better.
If you only take one thing from this: buy for the shape of the tank, not the litres on the label. A 200 litre cube and a 200 litre long tank are different engineering problems that happen to share a number.
Questions readers ask about this
A 120 cm tank usually needs one in-tank pump, occasionally two. A single circulation pump drives a visible current about 60 to 70 cm before it breaks down into drift, so one unit mounted at the far end from the filter output covers the run. Add a second only if a flake food test shows the middle third going slack.
No. Freshwater community tanks run happily at 4 to 6 times volume per hour, and pushing 15 times that stresses long finned fish and scours sand into bare patches. Reef systems are the exception, where SPS corals want 30 to 50 times. Match the number to the livestock, then make sure the pattern is a slow loop rather than a jet.
Yes, but at its real output rather than the box figure. A canister rated at 1450 litres per hour typically delivers somewhere between 800 and 1000 once media, hoses, and vertical lift are accounted for. Count that realistic number first, then add in-tank pumps to make up the difference to your target.
Size it on head height rather than volume. Measure the vertical lift from pump to outlet, allow roughly 30 cm of equivalent extra lift for each 90 degree elbow, then read the manufacturer head curve at that total. Aim for 5 to 10 times display volume actually delivered rather than rated.
Rarely in a long tank. A single 4000 litre per hour pump produces a fast jet in one place and leaves the rest of the tank to drift, while two 2000 litre per hour pumps set up a proper circuit. Two smaller units also fail more gracefully, since losing one leaves you with half the flow instead of none.
