I posted a while ago about my pleco who got sick and died. Now his tankmate, an oscar, is also looking very ill. His left eye is bulging outward, almost like it looks ready to fall clear out of his head (extremely disturbing to see). He is floating up at the top of the tank, but if he gets disturbed he starts violently seizing and flailing a foot or two in random directions, slamming into everything, and twitching until he stops moving.
I am pretty sure he won't last the night. I did a big water change, and have treated the tank with Kanaplex in the water (I had it already because I've used it in the past for my goldfish, and the package says it is good for "pop eye"), but I don't know what else can be done about it. The other tankmate is a pike cichlid who doesn't look as bad, but is unusually lethargic and hiding at the bottom.
My water parameters seem okay. Zero ammonia, zero nitrite, nitrates below 10 ppm. The pH last I checked a few days ago was at 7. The tank temperature is around 79-80 degrees (I don't have a heater in the tank, currently. I do own one and it does work, but I figured I wouldn't need it because the water is really warm anyway and doesn't vary by more than a couple degrees throughout the day).
Anyway, I really don't think he has a chance but I treated with the antibiotic because I would feel bad if I didn't. It is very upsetting for me to see this happen to these fish; they are rescues and I wanted to help them best I could, but it's not going well.
I'm sorry about your oscar. I'm also sorry that this new forum is not telling me there are new posts!
It sounds like he has popeye which means an internal bacterial infection.
I would expect there to be water issues with such problems but your water seems to be testing fine. It's been a few days since you posted. How are the fish now? A 50% water change with the addition of dechlorinator and some aquarium salt could help if there is in fact something wrong in the water that you have not detected (as long as it's not also in the tap water). Water changes are also a good first step when things are going wrong, and we don't know why.