Need help solving disease

[stellabl3u]

I decided to check out the local petco and ended up buying a bicolor and a royal gramma that day... that was less then two weeks ago and they have both passed away in the last 48 hours. Everyone else has been looking healthy, for the most part, when I added the new guys my tang got pretty stressed out at first, broke out with ich(or what I believed to be) well she has been doing a lot better after I did a few treatments with ich x on the tank, however a couple days ago I noticed my bi color hiding, I fed a bunch that night to see if she was eating or not (the gramma was hiding too but would come out for food). I found the bicolor dead on the sand bed the next morning of day 3 on ich x treatment, which isn’t copper based. So I thought at first maybe petco uses a copper product for their tanks, and I read it can take a few weeks for copper posing to show.. but then last night I noticed my gramma laying on the sand for a second, and then when I came home from work I couldn’t find her at first until I fed and just barely caught a glimpse of her moving thru the rock. Well maybe two hours later I found her dead on the sand... and in the last couple days I’ve noticed my tang less active then she was prior even after we started the treatment.. and now she has some serious spotage on her fins and her face looks like it’s peeling.. she’s breathing real heavy even thru her mouth, like blowing sand around occasionally. I haven’t notice any flashing from her whereas with the gramma I did. (Until I decided to video tape her just now!!)The bicolor I never noticed flashing either.. my tang looks really sick and I’m concerned it’s velvet or brook.. i have a sailfin tang that I took on due to someone having it in only a 30g tank.. she’s also breaking out with white spotsI’m going to upload a photo hopefully you guys can give me some answer and some advice.
I just checked my parameters
Nitrate: 20
Nitrite: .5
PH: 8.2
Ammonia: 0
Haven’t checked phosphate yet but I will when I get home.
livestock include:
Pair of clowns
Diamond goby
Watchman goby
Lime damsel
Sailfin
Tomini
Cleaner shrimp
A boat load of hermits
Pink tip Haiti anemone
Rock flower anemone
Random corals
Feather duster
A couple turbo snails
Another pair of clowns in the sump
System is a 75g with a 20g sump

 

[ReeferLou]

You will need a copper test kit to make sure the dose is correct over time. I recommend the API copper test kit - it is the lowest price copper test kit and you will need it to monitor the proper levels as you do water changes.

API Kit

Good Luck Sir!