Box GPH is a pump rating, not tank flow. Here is how to derate a filter for media, hose runs and lift, then confirm real turnover with a graduated bucket and a stopwatch before you buy.
Menu
Filter boxes sell a pump number. Your aquarium runs on a plumbing system. The gap between those two things is why a first-time buyer can end up with a filter printed "for tanks up to 55 gallons" that leaves mulm in the far corner of a 29.
On the fishgoo bench in Portland we measure flow by hand — graduated container, stopwatch, three timed catches — on day 1, day 14 and day 42 of every filter cycle. The day-1 number tells you what you bought. The day-42 number tells you what you will live with.
Your filter has two flow rates, and the seller shouts only one
Pump output is what the impeller moves with nothing attached to it. Filter circulation is what actually reaches the tank after water has been dragged through foam, rings and carbon, up a return hose and out a nozzle. Most brands publish only the first figure. The ones that publish both are instructive:
| Canister (120 V) | Pump output, as boxed | Filter circulation, media loaded | Share of the box | Max head pressure |
|---|---|---|---|---|
| Fluval 107 | 145 US GPH / 550 LPH | 95 US GPH / 360 LPH | 66% | 4.75 ft / 1.45 m |
| Fluval 207 | 206 US GPH / 780 LPH | 121 US GPH / 460 LPH | 59% | 4.75 ft / 1.45 m |
| Fluval 307 | 303 US GPH / 1,150 LPH | 206 US GPH / 780 LPH | 68% | 5.75 ft / 1.75 m |
| Fluval 407 | 383 US GPH / 1,450 LPH | 245 US GPH / 930 LPH | 64% | 7 ft / 2.25 m |
One pattern across four sizes: 59 to 68% of the headline number makes it into the tank with clean media and a tidy hose run. Eheim is blunter in its own service documentation, stating that filter throughput starts near half the pump rating and can fall below 40% once hoses and media have fouled. An older Hagen spec sheet for the 04 series listed a 340 GPH pump against 225 GPH of filter circulation, and noted that figure was measured with intake and return hoses of equal length — a condition almost no cabinet under a real tank meets.
Where the gallons go
- Media. Every sponge, floss pad and ring you stack is resistance. Fine mechanical media costs more flow than coarse foam at the same thickness, which is why a heavily packed 407 can measure like a lightly packed 307.
- Hose runs. Length, internal diameter and bends all charge you. In one measured hobbyist test, a 2,000 L/h canister delivered roughly 1,350 L/h straight off the U-bend, about 1,000 L/h with one spraybar module, and 800–900 L/h with two or three. Narrower aftermarket tubing does the same thing more quietly.
- Lift. For a sealed canister, static head is the vertical distance from the tank's water surface to the outlet — not from the tank down to the canister on the floor. The intake operates as a siphon, so parking the canister a metre lower inside the cabinet costs you almost nothing. Setting a spraybar six inches above the waterline does cost you real flow, and max head pressure (4.75–7 ft on the models above) is where flow stops entirely.
- Time. Impellers flatten, prefilters clog and hose walls film over. That is why we re-measure on day 14 and day 42 instead of trusting one clean-install reading.
Do the sizing math on real flow, not the box
Target 4–6x the tank's actual water volume per hour. Start by measuring that volume instead of trusting the label: inside length x width x water depth in inches, divided by 231, gives US gallons — then subtract substrate and hardscape displacement.
| Real water volume | 4x target | 6x target | Pump figure usually needed (media-loaded canister) |
|---|---|---|---|
| 20 gal / 76 L | 80 GPH / 300 LPH | 120 GPH / 455 LPH | ~120–200 GPH |
| 29 gal / 110 L | 116 GPH / 440 LPH | 174 GPH / 660 LPH | ~180–290 GPH |
| 40 gal / 151 L | 160 GPH / 605 LPH | 240 GPH / 910 LPH | ~250–400 GPH |
| 55 gal / 208 L | 220 GPH / 833 LPH | 330 GPH / 1,250 LPH | ~350–550 GPH |
| 75 gal / 284 L | 300 GPH / 1,135 LPH | 450 GPH / 1,700 LPH | ~470–750 GPH |
If a brand publishes a media-loaded figure, buy against that column and ignore the pump number entirely. If it publishes only pump output, assume 60–70% arrives at the tank and size accordingly. Messy keepers — goldfish, large cichlids, grow-out tanks — should sit at the 6x end of the range and above.
The fishgoo bucket-and-stopwatch protocol
- Use a graduated container read in quarts or litres. A 5-gallon bucket marked every half-gallon, or a 10-litre pail marked every 500 ml, beats a 2-litre jug every time.
- Run the filter exactly as installed: normal media, normal hose run, normal water level, outlet where it lives.
- Deflect the return into the bucket at the same instant you start the stopwatch. One minute. No topping up and no re-aiming midway.
- Read the volume. GPH = gallons caught x 60. LPH = litres caught x 60. (1 US gallon = 3.785 L.)
- Repeat three times and average. The first catch usually runs high because you mis-timed the start.
- Log it on day 1, day 14 and day 42, noting water temperature and the media you are running so the number means something later.
If the measured number still clears 4x your real water volume when the filter is two months dirty, you sized it correctly. If it only clears 4x on day 1, you bought one size down.
What to do with a number that comes in short
Before spending money: shorten and unbend the return hose, pull one spraybar module, brush the hoses and impeller housing, swap floss for coarser foam, and test without the intake prefilter. Those five moves recovered meaningful flow in our own rack tanks. If it is still under 4x, add a second filter or a circulation pump rather than buying a bigger canister that inherits the same plumbing.
And if you measure near 6x and your betta is pinned in a corner, that is a sizing error in the other direction — baffle the return or dial the pump back rather than accepting a current the fish cannot live in.
We are not linking a single model here. Nothing in this size class has finished a full 12-month retest window on our rack, and pointing you at one would be guessing with your money. The scores we stand behind live in the freshwater filter section of our gear index, where every entry carries the flow figures we measured ourselves.