I am wanting to know how many fish could thrive in a 29 gallon (110 L) tank. Dimensions of the tank are 30” in L x 12” in W x 18” in H (76 x 30 x 46 cm). These guys could include any live bearers like guppies, platy fish and molly fish. In the future I am going to buy live plants so my tank is fully live-planted and not semi-planted. I currently have 9 fish which are 2 algae eaters, 2 platies and 5 guppies. I am just wondering so I don’t accidentally overcrowd my tank to where it is unhealthy or could cause problems to the fish.

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    Hi, could you please include the dimensions of you aquarium (length x width x height)? It could be useful for people wanting to answer because it would let them know the surface area of water/air interface facilitating gas exchange. My point is, two aquariums could have the same volume, but different dimensions/shape and thus different surface area of water/air interface - aquarium with the larger one could house more fish.
    – lila
    Commented Mar 19, 2021 at 0:11
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    The dimensions are: 30” in L x 12” in W x 18” in H. Commented Mar 19, 2021 at 2:29
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    you can find some information about this here thesprucepets.com/amount-of-fish-and-aquarium-size-1378335 Commented Mar 19, 2021 at 5:33

1 Answer 1


The thumb rule is:

1 cm of fish for each 1 liter of water

This means:

  • 1 fish, 5 cm/fish -> 5 liters of water
  • 2 fishes, 2.5 cm/fish -> 5 liters of water

Now depending on which fish you want, and depending on their needs, you can make some estimation yourself.

As a kind reminder (it is sometimes forgotten): plants, filters, rocks do NOT count as water. With other words, take into consideration the water, and not the total volume of the aquarium.

Overcrowding can become a problem (the stress for the fish, the chemical balance of the water...). Under-crowding is unlikely to become a problem.

Also, do not forget that some fish prefer to live in schools (i.e. you need at least 4-5 of them, if you want them not stressed). Study the fish that you want to find out their particular needs.

I personally tend to overcrowd the aquariums, and I did not notice any problem. However, my behavior (and experience) should not be taken as science or as guarantee of success.

  • To this you can add that this rule tends to break down for larger fish. A better approximation would be based on volume, which increases by the third power (ie the cube). The fish volume of a 15 cm fish is very different than the volume of 15 1 cm fish. The latter works in a 50 l tank while the former does not. Commented Mar 25, 2021 at 16:02

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