If you only have a undergravel filter you are going to need power filters also. For a 75G you may want to look at buying 2 AquaClear 500's. They work pretty well and are a pretty good price. I assume you have enough heater if you had a Oscar so that should not be a issue. Most people keep P tanks around 80 degrees.KINGNIEVE said:75 GALLON TANK AND I HAVE AN UNDERGROUND FILTER!!