3-5 discus should be doable in a 55gal aquarium as long as water changes are kept up with. A general rule you can follow is one discus per 10 gallons. Since these fish are tall, do not keep them in shallow tanks.

In home aquariums, discus live for an average of 10 years, but can live up to 15 years, and can grow up to 8 inches. Like many fish in the home aquarium, they will eat almost anything that fits within their mouth. Due to their size, they often require a minimum 55 US gal (210 L) – 75 US gal (280 L) aquarium.

King of the Aquarium Wild caught discus can be challenging to keep, due to their specialized water chemistry and dietary needs, however, most discus sold today are captive bred and are much easier to keep. That said, they are not for beginners for a variety of reasons.

75- gallons is the most common tank used for discus fish stocking. Keep 1 discus fish per 10-gallon water. So, you can keep 6 or 7 discus fish in your 75-gallon tank.