Advise plz..

I have a 60 gal tank..I have a lion fish, Dogface puffer, Green wolf eel & a yellow tang. The other day I left the house for only a couple hrs came home and found my Yellow tang laying on his side breathing very fast with my lion fish hoovering over him?..took tang to pet store they said the tang might have aggervated the lion fish, & lion fish stung him..okay.. next day trader the lion fish in figuring that was the problem..& got a sailfin tang & Niger trigger ,inplace.I've been reading on the internet about tangs..& now im worried it was my oxygen level..not for sure..I've been watching my Niger T., he seems to be breathing a lil heavy, not a lot though..not sure if he suppose to do that or not?? so i added more ripple to surrface just in case..ANY ADVISE???
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You should do a whole lot more reading before you buy anything else at this point. Your tank is overstocked, with some fish that are iffy about going together to begin with.

We need to know parameters. Ammonia/Nitrite/Nitrate/Salinity/pH/Alk/Dissolved Oxygen (if possible). What kind of equipment do you have? fuge/sump/skimmer/filters?