If the tank is FOWLR would be hesitant for a whole tank treatment of hypo-salinitity due to the possible effect to life on the LR (pods, sponges, bacteria etc).

it will not affect bacteria.
probably would effect pods and sponge.

hea
 

howdy 20012002 is right about the hyposalanity it works. As of copper killing fish it only happens if you overdose.The ocean has a certain level of copper in it as does (THE REEF PLUS) .You should reed the labels on some additives as I am sure it will shock people and yes all fish have parasites they are just dorment waiting for the right time to come out.Stress and bad water are the biggest reasons.

A fifteen gallon aquarium housing 5 fish and other creatures means that you are overstocked, I think. I have five fish in a 29 gallon and I consider myself to be right at the limit. As others have said, the tangs are going to need a new home regardless.

Do some water changes and buy some ich medicine that doesn't contain metals. Most SW LFS have it available. Wais & Elite carry it for sure and I am sure Gold would too.
Now that said IME tangs are ich-magnets. They can develop ich spots just from stress and then they can be gone a few hours or days later. If you want to run a nano tank which I would consider both yours to be, you will have better success if you stock it accordingly. If you want more fish then you need to have a system with equipment that can handle the bioload and a fluval filter doesn't cut it. Sorry, you will have to spend some more $$$$ unless you stay with a FOWLR.

Almost all fish stores copper there fish only tanks as well as a lot of public aquariums with fish only displays as they cannot afford huge losses.

My 15 gallon tank seemed perfect though and my blue tang never seemed to have a problem. My yellow tang as of now doesn't seem to have any spots and the tank seems back to normal. Of course, I don't know that for sure though.

So should I just leave it with the 5 fish I have now? I do have 5 snails in the tank too.

And again, because I just wung this from the beginning, I have no idea what tests I should be doing and what kind of test kit to buy. I have always done my water changes though. What would be my best option? Also, Is my yellow tang suffering in any way by being "cooped" up in a 32 G then?

Thanks for all the advise. This site is great! And I would like to assure the guy wondering if I am some fake troll account trying to do something extraordinary besides obtaining advise on fish, that I am absolutely real and that I really really do just want fish advise!!!:)

How big are the tangs?

Welcome to Canreef. What are all the types of fish you have? The tangs need to go for sure unless they are really small but even then they will need a bigger home down the road. As for testing for starters you need to test for Nitrate, Ammonia, and Nitrite.