This looks to be ammonia burns. For cleaning your filter use tank water that you have siphoned out. Do not ever user tap water the chlorine insta-wipes your biological filter. Also use something like Seachem Prime when doing the water change. Oscars poop a ton and so do RTC. The ammonia seems to be climbing faster that the bioload can handle (if there is any bacteria). For now simple water change and proper filter maintenance can correct the issue and prevent further losses.
