75 gal seems too small. I am a firm believer that stress is the #1 killer of fish. I too had 2 healthy happy oscars in a 110 gal tank. When they hit 5 and 5.5 years they succumbed to 1. HITH and the second breathing issues and lethargy.
Tank always cleaned/ pristine low nitrate. Ph could have been part of it but I have finally come around to understand the stress factor more and more.
After what happened I would only keep oscars in a 200+ gal tank. Especially if there are tankmates.
Figure out the stress and 90% problems solved.