Stocking calculator
Build your stock list and check whether it actually fits your tank: bioload, filtration, minimum tank size, water parameters, schooling needs and temperament, all in one place.
Updated 18 June 2026
You'll get the most out of this tool with your tank's water volume and water parameters. Need them? Try the tank volume calculator, the hardness converter, and the water parameters guide.
Results
Compatibility is only partly checked. For an all-fish tank we cross-check temperament and known conflicts, but that's not the whole picture. Individual personalities, sex ratios, the order fish are added, and how the tank is planted all affect whether they settle. Read a clear result as "no obvious red flags", not a guarantee.
No problems detected for this combination. This is a guideline estimate, so always research each species and watch your tank's real behaviour, especially in the first few months.
Stocking estimates are guidelines, not guarantees. Bioload depends on feeding, planting, maintenance and individual fish. Treat anything over 85% as "stock conservatively and keep up with water changes".
How this calculator works
This tool checks your stock list against two separate questions:
- Stocking-level percentage. "Is there enough capacity for the waste?"
- Minimum tank size. "Is the tank physically big enough for these species at all?"
A single betta has a tiny bioload but still needs a tank of a sensible size to live well; the two numbers can disagree, and that's the point.
Stocking level (bioload)
Each animal is given a bioload estimate based on its adult length and diet. Waste output scales roughly with the square of body length, so a 10 cm fish carries far more load than three 3 cm fish. Carnivores and heavy-grazing herbivores are weighted slightly higher because they produce more waste, and invertebrates such as shrimp and snails are weighted much lower. A few notably messy fish, goldfish in particular, carry an extra weighting on top, because they produce far more waste than their length alone would suggest. The total is compared against your tank's water volume to give a percentage. Under 50% is lightly stocked, 50–85% is comfortable, 85–100% is full, and above 100% means you're asking a lot of your filter and maintenance routine.
Filter turnover
If you enter your filter's rated flow, the calculator shows turnover, the number of times per hour your filter could cycle the entire tank volume. A common target is 4–6× for lightly stocked tanks and 6–10× for busier ones. Rated flow is optimistic (media, head height and maintenance all reduce it in practice), so treat the figure as a ceiling.
Minimum tank size, schooling and temperament
The calculator flags any species whose own minimum tank size is larger than your tank, and any shoaling species stocked below its recommended group size. Selected species are also cross-checked against each other's known incompatibilities, with extra care around semi-aggressive fish and special cases such as keeping more than one male betta. For an all-fish tank we flag that this temperament check is only partial: individual personality, sex ratios, introduction order and aquascaping all matter, and none can be read from a species list.
Water fit and KH
The calculator flags any species whose temperature, pH or hardness range doesn't suit the water you entered. It also checks the group against itself: if your chosen species have no overlapping comfortable range (for example, mixing a cool-water species with a tropical one), it tells you, even if you haven't entered your own water yet. Carbonate hardness (KH) has no per-species value in our data, so it's used advisorily: very low KH warns about pH instability rather than ruling species in or out.
What this calculator does not do
It can't see your aquascape, your feeding habits, or the temperament of your individual fish. It assumes a reasonably planted, well-maintained freshwater community tank. Use it to sanity-check a plan and catch obvious problems early. For the rest, rely on the individual species care guides, our guide on how to stock a community tank, and your own observation. When in doubt, stock lightly and add livestock gradually so your filter can keep pace.