UK retail tank sizes and the Animal Welfare Act

UK pet shops commonly sell standard tank sizes such as 54, 64, and 125 litres, and the Animal Welfare Act 2006 places a legal duty of care on owners to provide an environment suitable for a fish's needs, which in practice usually means a bigger tank than the very small "starter" bowls once common, since most popular species need considerably more swimming space than a bowl provides.

The old "1 inch of fish per gallon" rule versus UK metric practice

Older fishkeeping guides quote "1 inch of fish per gallon of water", a US-imperial rule that does not translate cleanly to UK metric tank sizes or the centimetre-based rule of thumb used here, and both versions are rough approximations at best: an adult goldfish or a territorial cichlid needs vastly more space than a similarly-sized small community fish like a tetra.