Aquarium Stocking Planner

A balanced aquarium comes down to three things: how much bioload your water volume and filtration can safely process, whether each species' physical and social needs are met, and whether the fish sharing that water can actually tolerate each other. Stocking density is not just a headcount — a single 15cm fish can produce as much waste as a dozen 2cm tetras, so 'how many fish' is really a question about total adult biomass relative to filtration capacity and surface area for gas exchange. That is why this planner works in liters and converts species size and temperament into an estimated bioload rather than a flat fish-per-liter rule of thumb, and why the fill percentage should be read as a safety margin, not a target to max out.

Schooling needs are the most commonly overlooked factor in stocking mistakes. Species like neon tetras, corydoras, and many barbs evolved in large, loose groups; kept in twos or threes they experience chronic stress, which suppresses their immune system and shortens their lifespan even when water parameters are perfect. The planner flags a school-size warning whenever a shoaling species drops below its published minimum group size, because a healthy population number is as much a welfare requirement as a water-chemistry one.

Compatibility is where most beginner tanks go wrong, and it rarely comes down to a single obvious cause. Two peaceful, similarly-sized fish can still clash if their preferred temperature or pH ranges barely overlap, forcing a compromise that stresses both. A calm, slow-moving species can be harassed to death by a boisterous fin-nipper even if no one would call the nipper 'aggressive' in isolation. And predation risk is a matter of proportion, not intent — an otherwise mild-mannered fish will eat anything small enough to fit in its mouth once it's grown to adult size, which is why the planner checks adult size ratios, not just the size of the fish you buy today at the shop.

This is also why every verdict here comes with a stated reason rather than a bare pass or fail. 'Incompatible' on its own tells you nothing you can act on; knowing it is because two species' pH ranges do not overlap, or because one is a known fin-nipper, tells you exactly what to change — pick a different tankmate, or adjust your water chemistry before you buy. None of this replaces watching your own tank: cycle it fully before adding fish, introduce new species gradually, and keep monitoring ammonia, nitrite, and nitrate even after the numbers here look good. Published species data is a well-informed starting point, not a guarantee, and the individual fish in your tank will always have the final say.