How big are these fish?
You have way too many fish in there and the water quality will deteriorate making the fish succeptable to diseases, like ich.
What all have you tried to use, how long and how are you using it?
If you use carbon you need to take that out while treating.
Aquarisol, nox-ich and rid-ich are all good at getting rid of ich. Treat for at least 10-12 days even if you don't see anymore ich after a few days.
I would normally say to turn the temp up but with all those fish in that small of a tank, I wouldn't put any more undue stress on them than necessary right now.
At most, I would only keep 2 of those convicts in a 29g. Even 1 Oscar alone is too large and messy for a 29g.