How many fish can I have in a marine/reef tank?

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  • Graeme
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This has to be one of the most common questions asked, and indeed I asked it too (in fact I've asked it a few times).  So, I thought I'd start the topic off, and let people ask questions, and contribute.

Here's my current understanding...

It's not easy to answer this question, as there are so many variables.

As a general 'rule of thumb', it's often quoted that 1 inch (2.5cm) of fish (measured nose to base of tail) per 20lt is a good starting point.  This rule is flawed though, as there are issues just with this very simple sounding rule e.g. If I have a 200lt tank, I should be able to have 25cm worth of fish.  But, imagine the difference between the amount of food and waste to support say 10 x 2.5cm fish, versus that required to support 2 x 12.5cm fish.  You should consider the size the fish is likely to grow to in your tank; the shape of the fish (long and slim, or round and fat); is it a meat eater, omnivore, or algae eater?

You must also consider a lot of other factors, such as:
  • How mature the tank is - will give an idea of how good your biological filter is
  • How much liverock per litre of water you have - again, about the filtering capacity of your tank
  • What size skimmer you have
  • How wet you skim
  • How frequent and the percentage of water changes you do
  • Whether you have any nutrient export methods - such as cheato, algae scrubber, etc.
  • How effective your CUC is
  • What corals you have that will also help feed on leftovers, and contribute to the ecosystem
  • What corals you have that require additional, spot feeding, creating more waste
  • etc.

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