Good idea from everyone here. But I would still check that pH just in case. Admittedly the tank was neglected and a neglected tank can have a pH crash. That would cause the ammonia to rise although it would be less toxic than ammonia in a higher pH.

Obviously Oscars do not like a pH of 6 but if the pH was dropping gradually, they may have simply adapted to it. I think adding media would help and better maintenance but it only takes one second to check the pH just as eliminate that as a cause?

Good luck. Hope you get back on track.