How perfect? Ph? 7.9-8.3. Temp 80. No nitrate/ammonia?
Your fish may have ich. That parasite, which looks like grains of salt infesting fins and body, also hides in the gills, where it may prevent the fish from breathing properly. You have to get them out to a quarantine tank (you can borrow some tank water for them, or make up new salt water) and lower the salinity by the addition of fresh water. Consult in the disease forum: I'm number dyslexic and would not attempt to tell you the correct salinity for a hyposalinity treatment. DO NOT DO HYPO in your main tank: it will harm your rock and sandbed.