Inch per Gallon Rule
What is the Inch Per Gallon Rule?
This little phrase gets banded about whenever stocking ideas are discussed, however it has many flaws.
It is based on the idea that for every inch of fish (measured from the nose to the base of the tail) it should be allowed 4 Litres (1 US G.) of swimming space.
There are variants on this rule, such as inch per 8 Litres (2 US G.) or even cm per 1 Litres (0.3US G.).
So why doesn't it work?
Basing stocking solely on this "rule" could result in a 25.4cm (10") Oscar being housed in a 38 Litres (10 US G.) tank, when in reality such an Oscar would need more like 246-284 Litres (65-75 US G.) at least. It must be remembered that a large fish, such as big Cichlids and Goldfish, have far more mass and bioload than a fish that only stays 2.5cm (1")! And its for this very reason this rule cannot possible work solely for stocking a fish tank.
But can I use this rule?
This rule can be used, but with a lot of other restrictions and considerations. It can only be applied to small slim-bodied shoaling fish that stay 5.1cm (2") and under, such as Neon Tetras, these fish have low mass and low waste so will not have a huge impact on bioload. It could also only be applied to tanks 57 Litres (15 US G.) or larger, mainly to allow such small fish to have room to shoal (shoaling fish should be kept in groups of 6 or more) and to swim as often these fish are surprisingly active, a behaviour which would be prevented in smaller tanks.
If you are stocking larger tanks, or planning larger fish such as big Cichlids or Plecos or predatory fish like Bichirs then clearly this rule will be blown out of the water. Larger fish have many other needs that need to be met that should be considered as well as potential adult sizes, such as they may be very messy so need large volumes of water, or may be very territorial so cannot be housed with other fish, or they may be extremely active needing very long tanks, and so on and so forth.
If you are planning a tank, do not rely entirely on the size of a fish or any "tank calculator" to stock it, use common sense and species research first and foremost and the end result should be a happy, healthy and contented tank of fish.