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Overcrowding a tank is a quick way to kill fish. How many fish can comfortably live in a 50-gallon aquarium depends on the fish species, the type of aquarium and the size of the tank surface area.
Many people believe that one inch of fish can fit in one gallon of water, so that 50 one-inch fish could fit in a 50-gallon aquarium. This calculation does not work for brackish water or salt water aquariums, or tall, narrow aquariums.
Many fish grow more than one inch across. Always use the size of the adult fish when making calculations about how many fish can fit in a particular tank. Some aggressive species, like the oscar or velvet cichlid, need 55 gallons for just one adult.
Gravel, plants and decorations take up space in an aquarium, leaving less room for the fish and less oxygen content in the water. Only add one or two fish at a time to a tank for a multi-species tank. If the fish start dying, stop adding fish.
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