I have a nine year old Oscar that until a recent tank change has been okay and disease free. I went to a pet shop and told the lady what type of filtration I had and she told me that I was defeating the purpose of filtration by having an undergravel filter and a water filter (whisper 20 series) She told me that I should get rid of one or the other to keep the water clear. So when I did a 75% water chang I also pulled up the gravel filter and noticed how dingly the water was (almost black) I started putting fresh water in imideatly and changed the water filters and it was clear a little over an hour later. Well now my Oscar has a disintended stomach. I noticed just a few days ago that he/she was lying on the side of the tank at the botom, leaning. I have been taking note that for the past few weeks he/she has not been eating but just a bite here and there. I thought at first it was the temp of the tank because I do not have a water heater for the tank and it has been getting as low as 60 deg. I don't know if there is any cure for this other than taking he/she out of the tank and putting them into isolation and waiting to see what happens.....( I also have a 9yr old silver dollar and alage eater) I don't believe this could be pregency because the Silver dollar has started pecking at the oscar and where he would fight back he/she is now just sitting there floating.