Discus fish can grow very large. You can see how large the blue diamond discus are in the above photo. They look like dinner plates! It's important to have a large enough aquarium to raise your discus. Their water needs to be very clean. The general rule for housing discus is to have at least 10 gallons of water per 1 adult discus fish. Since discus are shoaling fish and do best in groups of 5 or more, the minimum size tank you should use is a 50 gallon aquarium.