tang killin tank

[sonic1634]

i bought my tank set it up and had if for about 9mts now and since than ive went throught 4!!! tangs and 1 angel all the other fish ive had since i started the tank it seems that i just have bad luck with those kinda fish my alk is usally around 2meq calcium 440 ph 8.4 temp 76 F he last died yeasterday i notice he had red gill and was breathing slowy so i put him into and quartine tank with new fresh batch of saltwater premixed and from the lfs add a stress coat and mediflix(??) the water was cold so i stuck a heater and got the water up to 80 F and he look like he was doing better he was swimming slowly but i thought that it was to hot (all the book ive read said that the water need to be around 75) so i backed off the heat the temp went down to 76 and he was dead what did i do wrong?