I'd up the water changes big time - try about 90g daily
for about a week
I change about 90-100g in my 125g Oscar tank every week or two - basically all but 5-6" of water.
I suspect that the 25% weekly hasn't been enough, especially if this is the tank with the big redtail cat (two big messy fish).
That should help get his appetite going. Then I would recommend feeding him freeze-dried krill that has been soaked in one of the new "edible" medications from Gel-tek :
http://www.drsfostersmith.com/Product/P ... 004+113021
I think local Petco's carry these now, although I've had luck with Gel-tek's original anti-bacterial food, which I think I got at Petsmart - http://www.drsfostersmith.com/Product/P ... 004+113021
(a triple sulfa and nitrofurazone blend)
I don't feed using the Gel-tec directions of feeding the food directly to the fish - I use the Gel-tec food to soak either floating pellets or (preferably) freeze-dried krill, and then feed those to the oscar - I found that otherwise, a big oscar will ignore the gel-tec food (its too small).
Of course, since feeder fish are a primary vector for infections like this, keep any feeders out of the tank for at least a few weeks.
A big advantage of using medicated foods is that you're targeting the fish, not the tank water - so you don't need to treat 125g of water - and you don't destroy your biofilter.
Its new, its improved (its still a work-in-progress)