Equipment
Filter size
Real-flow target and the higher rated GPH you should actually buy.
Box-rated GPH assumes open flow. We inflate the purchase recommendation so real-world flow after media and height still hits your turnover target.
Results
Target turnover
5×/ hour
Ideal real flow
200GPH
Minimum real flow
160GPH
Buy (rated GPH)
308GPH
Community tanks do well at about 4–6× turnover. Hang-on-back or canister filters both work; prioritize biological media volume.
How to use this
- Enter actual water volume and the stock style that best matches waste load and flow preference.
- Note the ideal real-world GPH (what you want after media) and the higher “buy” rating for shopping.
- Compare HOB, sponge, and canister options that meet or exceed the buy rating — then check media volume.
- After install, adjust spray bars / baffles so fish are not blasted while debris still reaches the intake.
Turnover math
- Ideal GPH ≈ gallons × turnover multiplier
- Community ~5× · Planted ~6× · Goldfish/cichlid ~8× · Shrimp ~4×
- Buy rating ≈ ideal ÷ 0.65 (media/head derate)
Community tanks — what to shop for
Real flow target is 5× volume. Buy rating is higher because the box GPH is measured with no media. Goldfish and large cichlids: multiply these buy numbers by ~1.6.
| Tank | Real GPH target | Shop for (rated) |
|---|---|---|
| 10 gallon | 50 | ~80 GPH HOB or a sponge |
| 20 long | 100 | ~150 GPH HOB |
| 29 gallon | 145 | ~220 GPH HOB or small canister |
| 40 breeder | 200 | ~300 GPH canister or oversized HOB |
| 55 gallon | 275 | ~420 GPH canister |
Worth knowing
- Biological media matters more than raw GPH once you clear a minimum flow threshold.
- Rinse mechanical media in old tank water so you do not kill the nitrifying colony.
- For planted tanks, aim flow across the length so CO₂ and ferts distribute evenly.
Usual mistakes
- Buying the cheapest filter that matches gallons on the box without checking real GPH.
- Over-cleaning all media at once and crashing the cycle.
- Strong flow with long-finned fish or shrimp — baffle it or choose a gentler return.
See also: Stocking · Tank Volume · Water Change