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How many Fish

Question: How many fish can I keep in my Tank ?

Answer: It all depends (my favourite answer) !

Ask this question on the internet and you will get all sorts of formula and rules, but here I am going to relate the formula that I use, it has served me well for over 50 years. It uses the surface area of the water to suggest how many inches of fish the tank can support.

  • Slim bodied such as tetras and minnows                     12 sq inches per inch of fish
  • Full bodied such as angels and gourami                      20 sq inches per inch of fish
  • Coldwater such as goldfish                                        30 sq inches per inch of fish
  • Marine fish                                                               48 sq inches per inch of fish

If for example we consider one of the typical beginner set ups between 50 – 70 litre the tank size is 24” (61cm) x 12” (30cm) giving a total surface area of 288 sq inch. If we translate this to fish we could keep 24” of Tetras or small Barbs, 15” of Gourami, 10” of Goldfish or only 6” of marine fish.

As I noted above, these are the suggested stocking levels, and relies on some very important assumptions, proper maintenance, and at the very least, some basic level of skill from the fish keeper. In a poorly maintained aquarium and a novice fish keeper you may as well half these stocking levels. What’s more, these figures relate to the adult size of the fish. After all, it’s pointless counting your Neon’s as ½” when in less than 6 months time they are going to be 1”.

As I have noted previously, (twice), these are suggested stocking levels.  In the real world there are simply too many variables to be precise. The fish, the food, the water parameters, even the mix of species and the skill of the fish keeper all have an effect on the health of the aquarium.

Just one note before you go: when we measure the length of a fish we do NOT include the tail.


Maroon Clownfish in an Anemone