Aquarium filter: bio-load and media surface area
A filter does not clean water: it is a home for bacteria and a mechanical trap. Media surface and nitrogen matter, litres per hour do not.
What happens inside the filter
The nitrogen cycle runs ammonia, then nitrite, then nitrate. The first two are toxic at tenths of a milligram per litre; the third feeds plants and drives algae once it piles up.
Bacteria do the work, and they live on the surface of the media, not in its volume. A 30 ppi sponge and ceramic rings of the same volume are therefore completely different filters, even when the box states the same number of litres for both.
Turnover
For most tanks the guideline is 2 to 4 tank volumes per hour. Turnover is not filtration, though. What counts is contact between water and media, how the flow is distributed, and aeration. A flow that is too strong in a planted tank strips CO₂ out of the water; too weak and stagnant zones appear behind the decor, where nothing gets cleaned.
That is why two canisters rated at the same litres per hour can behave very differently in two tanks of the same size.
Media surface area
Every medium has its own surface per litre. A sponge gives a lot of area for little money but clogs quickly. Ceramic and sintered glass pack more area into the same volume but need mechanical pre-filtration ahead of them. K1 in a moving bed cleans itself through motion.
The calculator converts media volume into surface area and compares that with the nitrogen load from food and from the fish. Surface area is the number the box never shows, which is why two filters are hard to compare by litres of media alone.
Food as a nitrogen source
Protein is about 16% nitrogen. A fish keeps roughly a quarter of it; the rest ends up in the water. One gram of food at 40% protein yields about 48 mg of nitrogen, and that is the number the bio-load calculation starts from.
How many fish a tank can carry
The answer comes from nitrogen, not from the old rule of a litre per centimetre of fish. What you get is a biological limit, not advice on behaviour: territory, schooling, swimming speed and compatibility sit outside the model.
The two questions stay separate: biology sets the ceiling, behaviour decides who can live under it.
What the calculation leaves out
- plants as consumers of nitrogen and phosphate;
- denitrification in anaerobic zones;
- aeration and oxygen saturation;
- flow and stagnant zones;
- the substrate as extra surface for bacteria.
In a heavily planted tank the plants can take a large share of the nitrate, and the same filter then behaves differently than it would in a bare tank with the same fish.
Calculator
The calculator takes your dimensions and works out glass, weight, stand and doses.
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