He doesn't look well at all He seems have fin rot, some irritation of his skin and is very thin. He also looks like he may have pop-eye, but photo isn't very clear. Are his eyes swollen at all?
Can you run a full set of tests and post results for ammonia, nitrite, nitrate, pH and gH in the tank please? Also the current temperature and a list of all inhabitants (species', sizes and numbers of each).
What do you feed him? How often do you do partial water change, how much each time and when was the last change? Have you added anything to the tank recently - livestock, plants or decor?
I use test strips all the time and they are all ok and recently took water to shop and all ok with there test kit. He is in a 330l and i change 15 % every sunday. He is in with jd.green terror. Baby jag. Baby texas and green severum
OK have you got actual results please? And can you tell me what and how much you're feeding?
15% weekly sounds a bit low, normally we recommend at least 25% a week with typical community fish - for big carnivores it may need to be more than that, but actual test results will help with that.
I do advise ditching the strips in favour of liquid tests which are more accurate and less expensive in the long run.
Other than that, the most obvious problem is your stock. Just the Oscar needs a 4ft x 2ft x 2ft tank on his own. Adding other large cichlids requires a much larger tank circa 8ft x 3ft x 2ft and of those you've added, only the Severum is a suitable tank mate. The others are all highly aggressive and not suited to a mixed tank of this size.
Please read these profiles and take note especially of tank sizes and behaviour/compatibility.
Ok thanks for the help. Cant tell you exact results as dont have liquid kit. I used esha bacteria and have read on oscar forums that you wont know the final result of if its working until at least a week after final treatment. Will look into ofloading some fish then