Ich I'm in trouble and it's my own fault. Not researching

[KevyMv]

I have a ninety gallon used tank got in september that came with 50pounds of live rock, Chocalate chip star fish, Coral bandit shrimp, and a clown. I then put a yellow tang in it after 3 months, and then a watchmant goby, and recently a coral beuty and then this past weekend a hippo tang. I noticed today the hippo looks to have ich. Then noticed salt grain on the coral beuty(eye) and finally with the night light noticed a bunch of grains of ich on the yellow tang. I going to set up a qurantine tank tomorrow or tanks but not sure how to go about it and how many tanks to buy and should I take out the starfish clown, shrimp, even if I have not yet seen the symptoms. How should I divide the tanks with who in what or can I put them all in a 10 or 20 gallon tank. Finally I know I messed up I thought a quarantine tank needed eveything the 90 had and knew that the smaller tank the harder to maintain...Please Help I love the hobby and don't want my hard earned money to go to waste becaue of my stupidity.

[KevyMv]

sorry I knew I forgot something. I just tested my water.
salinity 1.021
temp 80
amonia .25
nitrite 0
nitrate 2.5
Ph 8.4

[DragonForce]

This is a very common problem with tangs. Hopefully steve-s can chime in.