The Gorge - Flash Bristow's Website
Fish Size

How many fish can my tank support? What size of fish can I get?

Written in response to the question "I thought the volume of water in the tank dictated the maximum length of fish it would support?"

There are many factors which determine how many fish your tank can support. Let's start with some basic rules of thumb:

At the top end, 1" of tank length = 1" fish length
At the bottom end, 1 gallon of water = 1" fish length

(Fish length includes head and body, not tail, of full adult size.)

Where you fall between those two depends on other things like

  • filtration - are you just at spec, or over filtered?
  • fish body / demands - e.g. a catfish produces more poop than you'd expect so you might want to double requirements for them, but a neon tetra or other tiny fish doesn't make much demand on the filter so you could have more than you calculated

    Then you get into whether the fish types you've chosen need a long swimming space - some fish should never go in a tank less than 4 foot long. Alternatively some fish should only be kept in groups, and the full grown length of a whole group might be too much for your tank so you shouldn't keep *any* of that type.

    And then you need to worry about territories and be sure you only keep fish which don't need much space and won't squabble; regardless of physical size, filter load and swimming length requirements, some fish need more room as "theirs".

    Other fish may be cannibalistic - to take an extreme, your tank may technically support several piranhas, but if they are hungry you will end up with just one - albeit a fat one!

    But if you're just starting out and stick to fish which aren't territorial (community fish) and which don't get too long for your tank or need lots of swimming room - then use the two rules I gave you, plump for somewhere in the middle, and flavour to taste!

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    This page last updated: 01 September 2022

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