Oh dear.
I think its your water params hun.
You really need to get this stuff checked out before you own fish!
Anyway I suggest you keep all lights off for about a week. Do gentle water changes (20%) every night for a week. By gentle I mean, take the water out without much splashing around, and put new water in without to much splashing around. Make sure the water has been condition to get rid of heavy metals and chlorine. Make sure it is the same temperature as the water in your tank.
DO NOT remove these fish for these water changes, it will most likely kill them if you have lost 2 already. When doing larger water changes, I put them in a bucker (I wait for them to swim into a jug which gently goes in very large bucket) and never had problems, but then again I test my water very often, so they aren't stressed.
I suspect your ammonia levels are sky high, and this is toxic to fish. I think the cause is the tiny tank and not to mention an oscar being in there.
You need too get a 75 gallon tank (if these survive) at least, get internal, and external filteration, a sump would be even better. Filter the tank as if it was a 125 g and do regular gentle water changes. Don't overfeed.
Good luck dude I hope all goes well, and we all have to learn by our mistakes sometime, so don't think I'm flaming you.
