When is it safe?

[nycwaters]

I have a fish only 180 gallon tank ,, a few things have happen in the last 2 weeks .. one is that i treated my ick problem with STOP PARASITE and it seemed like it worked ... then i waited a week and bought a yellow tang and he looks sick with in 2 days notsure:...
my question i guess is how i know its safe to buy more fish ,, how can i test the water to see if theres any bad bacteria or parasite in the water?.. and why my other fish are still alive dragon wrasse, fox face tang, panther grouper, mexican wrasse and a 2 damsels:
thank you guys ....

 

[vtecbro007]

you cant test for ---- liek that i would suggest to do the medicine for about a month because icks dont go away so fast. give it atleast a month.