Great looking tank, you might have temp issues being so close to those windows, your heater/chiller will be working overtime, you also might think about upgrading lights, I thought I saw an anemone, and those need a higher par then 36w PC's.
How long has the tank been running, what are the water parameters, what are you using for filtration?
Could be that tangs are super stressed out for some reason for them to succumb to ich. Try using garlic to get them to eat real well, feed alot and include seaweed in their diet, do this to build up their immunity.
It's a great size for tangs too, long space to swim.
I have used attack ich in the past with great success, u can get it at PETSMART or PETCO.