We have 2 in our 200 gal, We have a Niger that we've had for about 1 year and a Huma Huma thats been in the tank for about 3 months. Like it was posted above, it all depends on the size of your tank, the size of the fish, what kind and what else you have in there. For a 75 gal I wouldn't have more then one.(A small one) They do grow very fast, and will need to be moved when they or it is bigger. Lisa