Ok, well before someone start's getting mad at you(yup it happens...), I'll warn you right away. You are *Way* overstocked. Gonna put this gently. Oscar's have a HUGE, and when I say huge, I mean Huge Bio load, and this might even be the source of the problem. Adding to that, two oscars together isn't an ideal mix at all. A jack dempsey and an Oscar fish isn't a good mix either, although people say it can work, the worst case scenario is that they kill eachother. If the pleco you have is a common pleco, the problem only get's worst, with it's huge bio-load, and it's tendency to "lick" off other fish's slime coat, witch could very well irritate the oscar's, and they will take care of the pleco, and trust me, it won't be pretty.
Now for the snapper fish, if we're talking about an actual one, those things get huge, talking 14", almost as big as the oscar's(~16") and require approximately 200 GALLONS o_o! Cat fish require immense aquariums(depending on the type) and usually have an immense bio-load! Reed fish are, aswell huge fish, and require at least 50 gallon's to themselves.
*EDIT* The filter you have has no where near the capacity, nor the GPH, nor the proper filtration "TECHNIQUE" to handle this much Bio-Load, witch is innapropriate for this tank size anyways.
You could not house half of these fish, not the third, not the quarter, not the fifth, ect... in a 60 gallon tank. Sorry to break it to you, but you have an emergency...:/