Update: I returned from vacation and all fish survived. My 4 Inch tiger Oscar is not looking good though. He is breathing heavy and not eating. It also seems like his fins are a little torn/ripped which is strange. Tigerbarbs, convict and BN look fine and very active.
Water parameters are good. Fully cycled with less than 10 ppm nitrates. I did a 50% water change, but it doesn't appear to have made the Oscar happy.
The temperature is 80 degrees and I added a little bit of aqua salt. Ph has been stable at 7.4
I'm not sure how to read this situation. The firemouth died almost 3 weeks ago. If he had bacteria/parasite, could that be affecting the Oscar after all that time?
No external signs of problems on the fish.
What do you recommend I do at this point?