I have 155 gal bow, fish only, wet / dry, skimmer, 25 watt UV, chiller (constant 76 degrees). Have five year old Hippo tang and and four year old purple Tang. Both fish were previously housed in 72 gal bow and developed lateral line disease. Within weeks of being transferred into 155 the lateral line itself regenerated on both fish. Both fish also had extensive color loss around face. Most color regenerated on purple tang but not on Hippo. Both fish have been in 155 for over two years now. I perform monthly water changes (45 gal) with Instant Ocean maintained at 1.21. I tried to raise salt to 1.24 but the Hippo developed a white "rash" which went away after being treated with melafix or penecillin. I feed a constant varied diet of - Formula one & two, frozen prawn, spirulina, homemade mixture of chopped seafood - clams, shrimp, scallops, dried seaweed sheets, romaine lettuce once every two weeks. I always add weekly trace replacement and monthly vitamin supplement. PH is at 8.4 alkalinity at 4 meg/l. My purple tang looks "dull" in color and the Hippo has not regenerated tissue color around face. What can I do to correct these problems????