Hey everyone, I have a big bummer and I have no idea what is causing it. I have a 125 gal reef tank with a 40 gal sump. I had a yellow tang and a purple tang with 2 clowns, cardinal, damsel, blenny, and a royal gamma. 2 days ago I noticed my purple was breathing extra heavy, still ate so I just monitored, next day I noticed my yellow was now breathing heavy and now neither would eat. Now today both are dead, about 24 hours after seeing the labored breathing. The tank is over 5 years old and ive had both fish for over 4 years, last addition was 6 months ago and was in QT for 21 days in copper and prazipro. I immediately did a water change in case something got spilled into the tank and ran carbon. I also put in airstones to increase the oxygen. the salinity is at 1.025 refractometer, checked ammonia and is at zero, all other levels are within range. no temp spikes nothing out of the ordinary, seems to only be affecting the tangs? all coral and inverts are good. I did chemiclean about 2 months ago to clear up some cyano, no issues then. No visual signs on the fish, no spots, did notice some redness at the tail section near death. its very depressing especially since I have no idea why! almost want to quit the hobby 
