[Snappy]

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.