One thing you will soon learn is that there are many opinions about how many fish you can keep in an aquarium. A wide variety of factors come into play here.

My goal is to give beginners a few more tips. But again, this is not the holy grail of filling aquariums. The best known guideline for novice aquarium owners is the “1cm rule.” This calls for one liter of water in the tank for every centimeter of fish. However, this rule is considered outdated. I remember being told about the “1cm rule” followed by the statement that “it was a VERY general rule so you shouldn’t follow it too closely.” What other things affect how many fish I can have I didn’t know at the time. I had no idea.

I wasn’t sure, hadn’t done enough research, and followed that rule approximately. I had a 56 liter aquarium with a goldfish, 2 corys, 2-3 swordtails and about 8 guppies. All these fish died and I felt terrible. Now I know that I just filled this aquarium very poorly. The maximum number of fish can be easily calculated. To do this, take the amount of water available and the edge length of the aquarium. Each fish, depending on its size and habits, must be given a certain amount of water to swim in.

Rule of thumb: The edge length of the aquarium should be 10 times as long and 5 times as wide as the body length of the longest fish. Now you have calculated the guideline for the recommended amount of water. Due to filling level, planting and decoration the value varies by 10 to 20 percent. So subtract 20% of the value and then apply it to your fish.

Rule of thumb for the number calculation: Tropical freshwater fish: 25 cm2 per centimeter of body length (without tail) Coldwater fish: 75 cm2 per centimeter of body length Tropical marine fish: 120 cm2 per centimeter of body length Tip: Inform yourself already when buying young fish about their later final size!