I have a similar situation. Four of my fish I've had for 4 years, two of them I've had for 2 years, the latest is three months. At one point I hadn't introduced any new fish for about 6 months. Within that time, I had a vicious outbreak of ich AND velvet with a secondary bacterial infection. My maroon clowns got hit first, and overnight the yellowtail damsel had no tailfin, and the large tang was losing his pectorals. All this happened shortly after my heater's thermostat malfunctioned, I had lent my protein skimmer to someone (I had only been running it once per week), and my reef keeper temp probe also malfunctioned to shut off the lights at 82 degrees! I don't know when the temp spike started but I caught it at almost 90 degrees when I happened to check the floating thermometer. I dipped them all in warm Las Vegas chlorinated tap water for 10 minutes each and transfered them to a 40g breeding tank I had set up in the garage previously. I lost one blenny out of six fish. After a month, I moved all but the clowns back to the DT. Any new fish I added (properly acclimated) died within weeks, always gasping on the tank bottom in the morning... NONE OF THE VETERANS SHOWED SYMPTOMS. I didn't want to break down the DT again! I added a neon goby. No problem. Added a cleaner wrasse. The orchid dotty back had territorial "issues" with it, got sick after never showing symptoms for 2 years! I treated him as before but I think the territory stress was too great and I removed him too late. He died in the hospital tank. The next morning I saw the wrasse and goby busy cleaning the tang that was completely covered with bumps. The bumps were gone by late afternoon. It has been 2 months since I added the last juvenile fish with only one hour drip acclimation followed by tap water bath. Everybody is fat and happy. It's obvious the tank is still infected, but the wrasse, goby, and now two cleaner shrimp are keeping it in check. No new fish for the DT-- just a new controller.:-)