I have a 90 gallon reef tank. My firefish has always been a little weak. He did go missing about 10 weeks ago for 1 week and reappeared with a missing eye. Otherwise, he was doing well. Yesterday I noticed his behavior changed. He began swimming to the other side of the tank. Today, he is just laying on the sand and breathing hard. He does go to eat food and will get out of the way if a hermit passes by him. His color looks good, he has no spots on him. pH is 8.2, salinity is 1.0235, nitrates, nitrites, and ammonia are all 0. All of my other fish are doing well. Any thoughts?