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How Many Fish

It’s always a worry that you might have populated your aquarium with too many fish. The only way to know exactly how many fish your tank can support would be to know gas-exchange rate of the tank and the biomass of the fish. Good luck with getting those values. Getting the biomass of the fish alone involves weighing them and calculating the gas-exchange rate of your aquarium involves some sophisticated equipment.

Luckily there is a fairly simple method of calculating fish capacity not involving weighing your fish which, if used sensibly, gives an accurate enough result. It uses the surface are of your tank and the total length of your fish. According to the fish type it uses a specific surface area per inch or cm of fish.

Fish Type Surface area per inch of fish Surface area per cm of fish Inches (cm) of fish in a 36″ x 12″ surface area tank Inches (cm) of fish in a bowl with an 8″ (20cm) diameter surface area
Slender Tropical 12 square inches per inch of fish 30 square cm per cm of fish 36″ (89cm) 4″ (10cm)
Full Bodied 20 square inches per inch of fish 50 square cm per cm of fish 22″ (54cm) 3″ (6cm)
Cold Water 30 square inches per inch of fish 76 square cm per cm of fish 14″ (36cm) 2″ (4cm)
Marine 48 square inches per inch of fish 122 square cm per cm of fish 9″ (22cm) 1″ (3cm)

Obviously you will have to modify the calculation for a mixture of slender and wide bodied fish. I’m afraid that there are a few provisos as well!

  1. These calculations assume the use of an air stone and/or filter
  2. You must use what will be the fully grown size of your fish – those you buy from the store are juveniles
  3. The results assume that the aquarium is properly maintained with regular partial water changes, monitored water chemistry and adequate filtration
  4. Schooling fish need a minimum number (usually 5) and may have to be excluded by the limit
  5. Very active or aggressive fish often need more space for their size

Finally, you should always stock a tank gradually when it is newly set up because you need to give the nitrogen cycle a chance to start working fully to get rid of ammonia and nitrites. The best approach would be to stock the tank in 4 stages over a period of several weeks, monitoring the water conditions very carefully over that period.

For the complete Freshwater Aquarium Guide see Katy’s Tropical Fish eBook.

Related posts:

  1. Freshwater Aquarium Fish Should Be Carefully Selected
  2. Modern Freshwater Aquarium Lighting
  3. Fish Disease – Prevention Rather than Cure
  4. Homepage
  5. Freshwater Aquarium Information

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Filed under: Freshwater Aquarium Maintenance • Freshwater Aquarium Setup

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