Well welcome to discus keeping! It’s not like any other fish keeping. 40g would make a good grow out tank. When grown a 75 would be better but hey a 90s almost the same foot print….. here’s my 40 qt tank with 2 4.5” discus
67432AD6-EE1E-4D6D-9664-A6BD0685EC9F.jpg black background makes it much darker! 2-3” discus will need 50% water changes daily and 4-6 feedings a day to grow out nice. Discus need water changes! They just do. Those in the 40 get 60% every other day.
Now this is a 50g with 3 6” discus
A8933430-F1F6-455B-BD96-324EAB7CDA43.jpg again qt tank and possibly breeder.
As a beginner you should go bare bottom and start with bigger fish. 4-5” discus. Don’t buy local by from recomended sellers from this site. In the long run there cheaper!!
Forget co2 and plants. Well plants maybe after a successful year in.