Filter capacity calculator
Enter the volume of each media type in your filter to find out its true biological capacity and how many grams of fish it can safely support at different water quality targets.
30ppi foam — 340 sq ft/ft³
20ppi foam / pot scrubbers / K1 static — 220–280 sq ft/ft³
K1 fluidised — 540 sq ft/ft³ (of total chamber volume)
Coarse foam / UGF gravel — 120–180 sq ft/ft³
Ceramic rings / lava rock / bio-balls — 8–12 sq ft/ft³ (from testing)
Source: aquariumscience.org
Full science at aquariumscience.org →
Flow rate vs biological surface area: which actually matters
Filter marketing focuses heavily on flow rate — the number of litres per hour the pump can push. This figure is useful for knowing how quickly water cycles through the filter and whether the turnover is adequate for gas exchange and waste pickup. But it tells you nothing about the filter's ability to process ammonia.
Ammonia is processed by nitrifying bacteria — specifically Nitrosomonas species converting ammonia to nitrite, and Nitrospira (including Comammox strains) converting nitrite to nitrate. These bacteria live on surfaces. The total effective surface area of your filter media determines how large a bacterial colony can be established, which determines how much ammonia the filter can oxidise per hour. A filter with 500 litres per hour of flow but only ceramic rings as media may process less ammonia than a filter with 200 litres per hour of flow packed with fine foam.
The key figure is effective surface area
Effective surface area accounts for how much of the media's total surface is actually accessible to bacteria and water flow. Fine 30ppi foam has a measured effective surface area roughly 30× higher than ceramic rings. A small volume of the right media outperforms a large volume of the wrong one.
Why ceramic rings underperform foam
Ceramic rings, sintered glass, and similar fired-clay media are marketed with impressive theoretical surface area figures — sometimes 300–500 m²/L based on total internal pore surface. The problem is that much of this surface is inside pores too small for bacteria to colonise, or inside dead-end passages that water flow cannot penetrate. Bacteria need both a surface to attach to and a supply of oxygenated, ammonia-bearing water to function.
Independent testing published by aquariumscience.org found that ceramic rings delivered an effective biological surface area of 8–12 sq ft per cubic foot of media. Fine 30ppi foam, by contrast, consistently delivered around 340 sq ft/ft³ — roughly 30 times more effective biological surface in the same volume of filter space. This means a single basket of fine foam in a canister filter typically outperforms a full filter canister packed with ceramic rings.
K1 media, when fluidised in a moving bed filter, performs best of all — around 540 sq ft/ft³ of chamber volume — because every grain is in constant motion, preventing surface fouling and keeping all surfaces accessible to bacteria and water flow.
Metabolic weight: why fish count is the wrong metric
The traditional "inch of fish per gallon" rule is a rough, outdated heuristic that fails in both directions. A single large fish produces far more ammonia than the rule suggests; a group of very small fish produces far less. The biologically meaningful unit is metabolic weight — the actual body mass of fish in your system, which correlates directly with ammonia production rate.
A 30 cm oscar weighs roughly 400–500 grams. Ten cardinal tetras together weigh approximately 20 grams. The oscar requires around 20–25 times the biological filtration of the entire tetra group. Stocking advice that says "one oscar is fine, it's just one fish" ignores the biomass comparison entirely. The calculator uses grams of fish as its output specifically to make this comparison concrete.
Use the fish species lookup to find typical maximum sizes for your planned fish, estimate their adult weights, and compare that to the gram capacity your filter can support at your chosen water quality target. Always calculate for adult size — fish grow, and an undersized filter for the adult stock is the most common cause of chronic water quality problems in established tanks.
For CO₂ and dosing calculations, use the tank volume calculator to get your accurate net water volume. For aquarium flow rate guidance, see the blog article on flow and the 10× rule.