I need some help. I have had these fish in QT for 3 weeks. Salinity has been at 1.010 for 2 weeks now. I had 5 yellow tangs, a bristle tooth,a tennentii, and the cleaner wrasse. They have all done very well this whole time. I did a 50% water change 2 days ago and another 5 gallon yesterday because I forgot to wash the sponge filter out the day before. I have about 40 gallons in a 55 gallon tank with a hydro sponge V rated for 125 gallons run with a mini jet. I also have a seperate airstone in the tank. Every body looked great last night. When I turned the lights on this morning 2 tangs were dead, everbody breathing very heavy and these brown patches on their sides. It is not as noticable on the yellow tangs but you can kind of get the idea fom the photos what I mean. The yellow tangs also have somewhat of a bloody appearence to their skin almost like vericose veins. The only fish not effected is the cleaner wrasse. I have had this happen another time with 2 other tangs. They made it through QT the salinity had been normal for about 3 weeks and I was just about to add them to the tank when this very thing happened. I have ammonia badges on the tanks to constantly moniter and their has never been a trace. PH is 8.14. I have been feeding new life spectrum pellets the whole time and they have all been eating like pigs. I did feed the anti parasite version (heavy on the garlic) yesterday. I dont know if that is a peice to the puzzle or not but I was feeding that pretty heavy the last time this happened. 

Thanks   

Damon

A couple things here:  When you say you forgot to wash the sponge filter - do you mean you physically clean/soak the sponge filter?

What are the water parameters - measured on test kit?  I DO NOT trust those ammonia alert monitors - I have had personal experience with brand new ones  not "alerting" and the ammonia being toxic....

How do you make up your water for water changes?  What type of water do you use?  How long do you aerate/mix the salt before adding to the tank?