Plan for adult size, not purchase size
Almost everything sold in an aquarium shop is a juvenile. The question is never whether the animal fits the tank today; it is whether the adult will have room to behave normally.
Welfare standard
A betta survives in a jar. A goldfish survives in a bowl for a while. A common pleco survives in a 60 litre tank until it is 35 cm long and cannot turn round. In each case the animal is alive, and in each case the answer to the question actually being asked is no.
AquariumHQ does not publish a capacity figure
Not because it is difficult, but because “how many animals will fit” is the wrong question. The stocking assessor returns a list of specific constraints with the reason behind each, so a keeper can see WHICH one binds — and there is no field anywhere in the model that holds a maximum number of fish.
The principles
Dimensions matter as much as volume
Volume tells you about dilution and oxygen. Length and width tell you whether a fish can swim, whether two territories can exist, and whether a bottom-dweller has floor space. A tall 100 L tank and a long 100 L tank are different homes.
Group requirements are requirements
Shoaling species held below their threshold show measurable, sustained stress behaviour — reduced feeding, colour loss, hiding, and in several species increased aggression towards other fish. A group of three of a species that needs ten is not a small group; it is the wrong environment.
Stability beats ideal numbers
A steady pH of 7.6 is better than a pH swinging between 6.8 and 7.4 in pursuit of a figure from a care sheet. Most parameter chasing does more harm than the parameter did.
A minimum is a floor, not a recommendation
Where AquariumHQ states a minimum tank size, it is the smallest environment in which the animal can express normal behaviour — not a target to aim at, and not a size at which the animal thrives.
Say when we do not know
Much husbandry guidance is convergent practice rather than measurement. Where that is true, we label it. Where sources disagree, we show the disagreement instead of averaging it away.
Describe behaviour, do not narrate feelings
Fish show stress responses that are measurable and behaviours that are observable. Claims about what an animal wants or enjoys are not supportable and are not made.
Bowls, vases and jars
Unheated, unfiltered containers — bowls, vases, jars and small display cubes — cannot hold stable temperature or stable water chemistry, and a container under about 15–20 litres swings too fast for any correction to keep up. No fish is a suitable occupant for one. Some are sold specifically for betta, which tolerate low oxygen through their labyrinth organ; tolerating low oxygen is not the same as needing less space, and a betta in a heated, filtered tank of adequate size behaves like a different animal.
The inch-per-gallon rule, and why it fails
The rule is discussed here because people arrive having heard it, and explaining why it fails is more useful than pretending it does not exist. Nothing on AquariumHQ uses it to produce advice.
Worked example
A 76 litre tank is about 20.1 US gallons. Applied to a 25 cm fish, the rule “permits” 2. Here is why that number is meaningless:
- The rule is linear in length, but waste output scales with roughly the cube of it. One 25 cm fish and ten 2.5 cm fish are the same "25 inches" to the rule and are not remotely the same load.
- It ignores the footprint. 2 fish of this length in a tall, narrow tank is a different proposition from the same fish in a long one.
- It ignores social requirements. It will happily permit one of a species that needs a group of twelve.
- It ignores territory, so it permits combinations that fail on behaviour long before they fail on water quality.
- It ignores oxygenation, which is a function of surface area and temperature, not volume.
- It was formulated for small community fish and produces absurd results outside that range — it "permits" a single adult common pleco in a 40 L tank.
Practices AquariumHQ does not recommend
| Practice | Why not |
|---|---|
| Unheated, unfiltered bowls and vases | Cannot hold stable temperature or chemistry. No fish is a suitable occupant. |
| Dyed or injected fish | The colour is applied by injection or dipping, causes injury and mortality, and fades. |
| Deformity strains | Balloon-bodied fish, bubble-eye goldfish and similar are bred for traits that impair swimming, vision or organ function. |
| Buying an animal for a job | An 'algae eater' or a 'clean-up crew' is an animal with its own requirements, and the job is usually a symptom of something else. |
| Stocking to a capacity figure | Optimises for the maximum that will survive rather than for the conditions the animals need. |
Never release aquarium life into the wild
Aquarium animals, plants, and aquarium water must never be released into ponds, rivers, canals, drains or the sea — not even in a country where the species is sold freely. Released fish and invertebrates establish populations that displace native species, and released plants and water carry pathogens, parasites and fragments capable of spreading without any animal at all. If you can no longer keep an animal, rehome it through an aquarium society, a retailer that accepts returns, or another keeper.
- Rehome through a local aquarium society, a specialist retailer or another keeper.
- Unwanted plants go in household waste or a hot compost heap, never in a watercourse or a garden pond.
- Never empty a bucket of tank water into a drain that discharges to open water.
- If an animal is beyond rehoming, ask an aquatic veterinarian about humane euthanasia rather than releasing it.