There is more to fishkeeping than "clear" water. To have healthy fish, a fishkeeper must understand the nitrogen cycle.
Basically, ammonia is produced in fish waste and respiration, and from food debris in the tank. There are beneficial bacteria in the filtration system that consume the ammonia and produce nitrites. Another group of beneficial bacteria convert the nitrites into nitrates.
Ammonia and nitrites are very toxic to fish. Nitrates are not as toxic, but are still bad for fish. When a tank is "cycled", there is enough beneficial bacteria in the filtration system to convert all ammonia and subsequent nitrites into nitrates. Having adequate filtration will result in zero ammonia and nitrites. However, nitrates will continue building up in the tank. Unless the tank is heavily planted, or you have a specialized nitrate removing system, the best way to reduce nitrates is with water changes.
While nitrates are not as toxic as ammonia and nitrites, high levels have been implicated in stunting, failure to thrive, hole-in-the-head disease (HITH) and shortened life span. For most fish, nitrates no higher than 20 part per million is recommended. The only way to know your water parameters is by testing the water. You should get a fresh water master test kit and test your water on a regular basis. That is the only way to know what is going on in your tank. Ammonia and nitrite readings should always be "0", and try to keep the nitrates less than 20 ppm. How much water you change and how frequent you do water changes should depend on how quickly the nitrates rise in the tank....you can have ten filters on your tank, and that will not reduct nitrates.
The reason people are saying your tank is overcrowded, is because that many large fish in such a small tank will result in high nitrate levels very quickly....you would probably have to do 100% water changes every day to keep the nitrates down.....that is what we had to do. My husband brought home a 55 gallon tank with 8 large fish in it, including 4 oscars. One oscar was already suffering from HITH, and he died soon after. Once we learned about the nitrogen cycle, we had to change the equivalent of 100% of the water every day just to get the nitrates down.
From personal experience, I would say that it is impossible to control the nitrate levels in your tank with that many fish (it could be done with a much larger tank where the nitrates would be diluted) Because oscars produce so much waste, one oscar alone in a 55 gallon tank is the usual recommendation.
In the 4.5 years we've had our fish, the four oscars and two plecos have died (they were all stunted from living most of their lives in bad water conditions). Only the two pacu are left. We upgraded from the 55 gallon tank to a 300 gallon tank, and are going to an 1800 gallon tank for them. Going much bigger should make it easier to keep the nitrates low.
Another thing---if you are buying feeder fish, and not quarantining them, you could introduce disease into your tank.
