Let your tank sit without any fish for six weeks. Any ich parasites will die in that time without a host.
Make the investment in an API Saltwater Master Test Kit. You can mail ordser one for around $20 or pick it up ina store for around $32. That will get you pH, ammonia, nitrite and nitrate test kits which you should have anyway. I'd also buy a refractometer to check your specific gravity (salinity) if you don't have one.
Once you have your parameters under control, an occellaris (sometime called a false percula) clown would be a good fisrt fish. True perculas IME are a little more delicate.
Please don't get any more tangs in a tank that small. They are very active fish that need a lot of room to swim and stress easily, making disease more likely. You have now experienced this first hand. Also, I would advise against any cleaner wrasses (the black and blue fish you had). They generally don't do well long term in captivity.
What kind of lighting do you have for your anemone? Please specify bulb type and wattage.
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