The water that moves is the tank's health

In an aquarium the filter is not an accessory: it is the liver. Every litre that passes through it delivers ammonia to the bacteria that transform it, and how many times per hour the whole tank crosses the filter — the turnover — is the number that separates living water from water on the defensive. But whoever sizes the filter on the box makes the classic mistake: the rated flow is measured on a bare pump, without media, without head height, without tubing. This calculator starts from the nameplate, subtracts the real losses and judges the turnover with two lenses: the current the filter moves and the fish load the tank asks it to carry.

The formula: the nameplate, the losses and the true turnovers

The chain is short and honest: Q_r = Q·(1 − h/100) is the real flow — the nameplate cleaned of loaded media, head height and elbows — and r = Q_r/V are the true hourly turnovers: how many times the tank's water passes through the filter in one hour. From there the two talking numbers: the minutes per full pass t = 60·V/Q_r, and the second lens ρ = d/V — the centimetres of fish per litre, the biological load that decides whether the turnovers truly suffice. The filter is judged with both: the first says how hard it works, the second for whom.

The volume scale: the same pump, five tanks

A pump rated 1,000 litres per hour, losses at 25 percent — 750 real litres — and 60 centimetres of fish; only the tank changes:

The flow column does not appear because it does not move: 750 real litres for everyone. That is the table's lesson: turnover is not a property of the pump, it is a property of the pump-tank pair — the same machine that floods the nano like a river gasps on the big tank. And the first two rows also light the second lens: 60 centimetres of fish in 40 or 60 litres is a crowded tank, whatever filter is running.

A complete numerical example

The typical community: a 120-litre tank, an external filter with a pump rated 1,000 litres per hour, losses at 25 percent — media in service, a metre of head, two elbows — and 60 centimetres of adult fish: a school of tetras, a few corydoras, a pair of gouramis. The real flow is 1,000 × 0.75 = 750 litres per hour; the true turnovers are 750/120 = 6.25 per hour — one full pass every 9.6 minutes — with 0.50 centimetres of fish per litre. Band c, the ideal band: the biology receives enough water and the current stays livable for fins and plants.

How to read the four turnover bands

Below 3 turnovers the filter gasps: it receives less water than the tank dirties, and the chemistry notices before the glass does. Between 3 and 5, the bare minimum: it holds with light stocking and punctual maintenance, without margin. Between 5 and 8, the ideal band of the community aquarium: enough water for the biology, livable current for the fish. From 8 up, river current: perfect for stream biotopes and rheophilic cichlids, tiring for bettas, guppies and slow-water plants. The right reading is not "more is better": it is the right current for the landscape you chose — and the borders of 3, 5 and 8 are touched exactly, as the tables show.

The flow loss: where the nameplate ends

The variable h is the most underrated in the bill — and the only one that worsens on its own. The nameplate is measured on a bare pump; in the tank, working against it, are the media that load up — a mature filter loses flow every week — the head height between pump and surface (every centimetre of climb is flow gifted to gravity), the elbows and undersized tubing, the dirty impeller. Physiology is 20-30 percent on a well-kept setup; a neglected filter reaches 50-60. The operative rule: turnover is designed on real flow, and real flow is measured — one litre counted at the outlet is worth more than any box.

The centimetre per litre: the second lens

Turnover says how hard the filter works; the load says for whom. The old rule of one centimetre of adult fish per litre remains community fishkeeping's most honest compass: below 0.5 the tank breathes, towards 1 the load gets serious, and from 1 centimetre per litre up the calculator flags it — the crowded tank, where no turnover saves the biology from the cause. It is the number to use before buying pumps: less fish, more litres or more water changes — because the best filter in the catalogue is still a bigger tank, and the second best is the Sunday siphon. The count is made on adult centimetres: the fairground goldfish does not stay four fingers long.

Litres or gallons: the same rule on both shores

The model is universal by construction: litres, litres-per-hour and centimetres carry no passport, and hourly turnover is a pure ratio that holds identically in gallons — the Anglo-Saxon "4× to 10× turnover" rule is exactly the same arithmetic in other units, and "inch per gallon" is the imperial cousin of the centimetre per litre (one inch per gallon ≈ 0.67 centimetres per litre: the American rule is even more prudent). The nameplates change — GPH on American pumps, l/h on European ones — but the calculator's question stays the same on every shore: how many times per hour does your water really pass through the filter.

Model limits and correct use

This is the hydraulic bill of turnover: it does not model the quality of filter media — one litre of good ceramic beats two of tired sponge — nor the specific chemistry of marine and shrimp tanks, which have rules of their own. Stocking in centimetres is a compass for freshwater communities, not a law: one plecostomus is not ten neons. And the filter is the continuous load par excellence: it runs 24 hours a day, 365 days a year — its place on the bill is told by the standby power calculator, and the other water in the house that asks for watts and patience has its own bill in the hot tub heating time calculator. Confirmation belongs to the real tank: flow measured at the outlet and water tests in hand.

The loss scale: the same pump, five setups

Tank fixed — 120 litres, a pump rated 1,000, 60 centimetres of fish — and a setup that ages: from the bare pump to the neglected filter.

The box's row comes first and exists in no tank: even the new setup takes 15 off it. The 40 percent row is the most instructive: the dirty sponge takes you exactly to the 5-turnover border — the same tank that lives in the ideal band when well kept slides onto the border through overdue maintenance alone, and at 60 it is already in the minimum. Loss is the only input that worsens by itself: the calendar works for it.

The stocking scale: the same filter, five verdicts

Tank and filter still — 120 litres, 750 real litres, 6.25 turnovers — and only the centimetres of fish growing:

The turnover column does not move — 6.25, always — because the filter does not know how many fish swim above it. Yet the verdict changes: at exactly 120 centimetres — one centimetre per litre — the flag fires to the millimetre, and from there up the same 6.25 turnovers serve a tank asking more than the biology can give. The verdict lives in the ratio between load and litres, not in the pump: it is the table to look at before every new purchase at the shop.

The borders that are touched

The three turnover borders are not roundings: they are reached exactly, and with round minutes. The 200-litre tank with 600 real litres runs at exactly 3.00 turnovers — one pass every 20.0 minutes, the entrance of the minimum. The 120-litre community with 600 real litres makes 5.00 — a 12.0-minute pass, the door of the ideal band. The 100-litre tank with 800 real litres touches 8.00 — a 7.5-minute pass, the threshold of river current. And the second lens has its equally exact threshold: one centimetre per litre, touched twice in the tables — the 60-litre cube with 60 centimetres and the community with 120 — while at 110 centimetres (0.92) the flag still stays silent. The calculator works on integers: the borders fall where declared.

Internal, external or sump: where the losses are born

The turnover bill is identical for every architecture; what changes is where the setup loses. The internal filter is the most honest with the nameplate — no head, short tubing — but it steals litres from the tank and space from the landscape. The external under the cabinet pays a good metre of climb and two elbows — the 20-30 physiology — but repays with litres of filter media the internal can only dream of. The sump thinks like a plant: the overflow is free, the loss lives entirely in the return pump — and that is where it is measured. The transversal rule: every bend in the tubing is flow given away — the short, straight path is the cheapest upgrade in the catalogue.

The mistakes the water does not forgive

The first is believing the box: sizing on the nameplate means living one band below the designed one — the loss table shows it row by row. The second is the filter switched off at night for noise or savings: the biology breathes oxygen and dies within hours in still water — it is the beginners' silent massacre, and it is prevented by choosing quiet pumps, not switches. The third is rinsing under the tap: chlorine wipes in one minute the colony that took a month to mature — media are rinsed in water taken from the tank, half at a time. The last is brutal current on the wrong fish: a betta in a nano with 18 turnovers lives in a perpetual storm — the right filter for the landscape, always.

How to gain turnovers without buying pumps

The good levers sit in maintenance and in the path. The monthly bucket method: flow measured at the outlet is the setup's thermometer — when it drops 20 from last month, the cleaning calendar has arrived on its own. Alternating maintenance: half the media at a time, on alternate weeks, keeps the flow high without ever zeroing the biology. The short path: one eliminated elbow and tubing of the right diameter give back flow points for free. And when the pump must be replaced anyway, the catalogue's rule: buy the size above — the pump working at 70 percent lives longer, makes less noise and leaves the media room to load up in peace.

Frequently asked questions

How many turnovers per hour does an aquarium need?

The ideal band for a freshwater community is 5-8 real turnovers per hour; 3-5 is the minimum that holds with light stocking; below 3 the filter gasps; above 8 it is river current — perfect for stream biotopes, tiring for bettas and guppies. The example community runs at 6.25.

Why is the rated flow not enough?

Because it is measured on a bare pump: in the tank, loaded media, head height, elbows and tubing work against it. Physiology is losing 20-30 percent on a well-kept setup — the 1,000 l/h pump moving 750 — and a neglected filter reaches 50-60. Turnover is counted on the water that actually moves.

How do I measure my filter's real flow?

With the bucket method: filter outlet into a graduated container for a timed interval, then multiply. One counted litre is worth more than any box — and repeated monthly it tells the ageing of the media better than any calendar.

What is the centimetre-of-fish-per-litre rule?

The historic compass of freshwater communities: below 0.5 cm of adult fish per litre the tank breathes, towards 1 the load is serious, from 1 up the calculator flags the crowded tank — where no turnover saves the biology. Count adult centimetres, not shop centimetres.

Can a filter be too powerful?

Yes, for the wrong landscape: beyond 8 turnovers the water runs like a stream — ideal for rheophilic cichlids and barbs, exhausting for bettas, guppies and slow-water plants. If the fish hold station behind the decor seeking shelter, you need a spray bar, an outlet deflector or a better-mannered pump.

How often should the filter be cleaned?

When the flow drops, not when the calendar says so: the monthly bucket method is the best warning light. Rinse media in tank water — never under the tap, chlorine wipes the biology — and never all at once: half at a time, on alternate weeks.

Should the filter be switched off at night?

Never: the biology breathes oxygen and dies within hours in still water — the filter switched off at night is the beginners' silent massacre. The filter is a 365-day continuous load: choose it efficient, not intermittent.

Does the rule work in gallons too?

Yes, identically: hourly turnover is a pure ratio. The Anglo-Saxon 4×-10× turnover rule is the same arithmetic in GPH, and inch-per-gallon is the imperial cousin of the centimetre per litre (1 inch/gallon ≈ 0.67 cm/litre). The nameplates change, the question does not.

Internal, external or sump: does the bill change?

The turnover bill is the same; the losses change. The internal loses little (no head) but steals litres from the tank; the external pays the climb and the tubing but filters more and better; the sump adds overflow and a return pump — there the loss is computed on the real return height.

How much does head height weigh on flow?

It is the steepest item: every pump has its curve, and flow collapses as the climb approaches the maximum head. The external filter under the cabinet pays a good metre of column: that is why the same pump performs differently in two setups.

Does turnover replace water changes?

No: the filter transforms ammonia, it does not remove the final nitrates nor replenish minerals. The Sunday siphon remains the catalogue's second-best filter — and with a crowded tank (from 1 cm/litre up) it becomes the first: more frequent, not more abundant.

What does the calculation NOT cover?

The quality of filter media, the chemistry of marine and shrimp tanks, surface oxygenation, heater sizing. It is the hydraulic bill of turnover: confirmation belongs to the real tank — flow measured at the outlet and water tests in hand.

Updated July 2026