The problem with oscars is that they are very messy fish and get large. If you don't have one, get a freshwater master test kit and routinely test for ammonia, nitrite and nitrates. In a cycled tank, the ammonia and nitrite readings are 0. The nitrates should be kept below 20 ppm. Water changes are the easiest way to keep the nitrates low. When you have to do massive frequent water changes to keep the nitrates low, you will know it's time to either get a larger tank, or remove some oscars. 55 gallons is the minimum recommended tank size for ONE oscar. High nitrates can lead to stunting, HITH, premature death of your oscars. How much and how frequently you change the water will depend on the nitrate readings