How Many Fish?

In case you haven't heard, there is this stupid "rule of thumb" in the aquarium hobby that you can keep 1 cm of fish for every 2 liters (1 inch per gallon) of water. Let me start by explaining why that rule should be ignored completely.

Consider a 3 cm (1 in.) Neon Tetra and a 3 cm Goldfish. In terms of waste produced and body mass, it will take about 20 Neons to equal that 1 Goldfish. Also consider a 3 cm Pink Kissing Gourami. The Pink Kisser is just a baby while the Neon is an adult already. That Kisser has a much higher metabolism and therefore will eat more and produce more waste. Also, the Kisser has much more body mass than a Neon. The cm/liter (in./gal.) rule is meant to apply to small fish at their adult size, such as Neons, Cherry Barbs, Cardinal Tetras, White Clouds, and Zebra Danios. Do NOT use this rule for any fish which is bigger than a Neon. Not even for a Platy or a Swordtail.

Now that you can see what I'm talking about, let me explain how many fish you can comfortably fit into your tank. The most important consideration is that you are doing regular water changes and have the appropriate biological filter for your tank size. Provided you are doing the appropriate maintenance and have the correct sized filter, then you will probably never have problems with water quality, regardless of how many fish you put in the tank.

The important considerations are aggression and swimming space. You should never put a fast swimming schooling fish in an aquarium smaller than 200 liters. Many hobbyists assume that they are losing fish mysteriously because the tank is "overcrowded." In reality they are just not giving the fish the appropriate swimming space. When packed into a small tank, "fin nippers" will become very territorial and create stress on your other fish. This is what is resulting in the mysterious fish loss. And it is not related to water quality.

OK, so how many fish? To be honest, there is not a limit. As long as you are putting the fish in the appropriate size aquarium, you will be content with the fish load long before you have reached the biological limits of the tank. I have kept 40 liter aquariums with 30 fish and 200 liter aquariums with 100 fish. The catch is to provide the correct maintenance and be smart about what fish you mix together. Just watch your tank. When your tank starts to look crowded then stop adding fish. There really is no "rule of thumb" for adding fish. This depends entirely on the mix of fish you choose. Good luck!

With my experience with amount of fish you can put in a tank is determined by two things: how much maintenance you’re willing to do, and fish compatibility/crowding. For example, you could pack 100 guppies in a 20 L tank (Don't try this at home, it's unfair to the fish) but you would probably have to do multiple water changes a day to keep them alive. When you want to keep a lot of fish in a small area you have to be vigilant of water quality, ammonia spikes, high nitrite/nitrates, and problems with bacterial levels which are imminent and deadly. Overcrowing fish (as pretty as it may look) is a bad idea, first you're likely to have many casualties along the way, not to mention your own stress of constant maintenance and as soon as you slack on the maintenance the whole operation will end up as lot of dead fish and a messed up bacterial bed. My approach to adding fish and finding the right balance is 'A Few at a Time' and 'Very Slowly'. By adding fish in slowly (2-4 week wait in-between per group for an established aquarium) you can better observe the effect of that 'one more fish' into your aquarium. If fish start looking unwell, or aggression becomes a problem you may be at your limit. You’ll also be able to see and correct any problems that may arise from the new individuals. Also, the “rule of thumb” of 1 cm of fish for every 2 liters of water should be completely ignored.

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