Help with Brown Scopus Tang

[780Matt]

i am a new reefer. I have in my tank 2 four striped damsel with is healthy and doing good. I bought my tang 2 weeks ago. He was eating well. I noticed this morning that he had some discolouration on him/her I didnt know what it was. I now found him on his side at the bottom of the tank barely moving. I moved him into another bucket away from my exsisting tank. It is a 25 gallon. I do not have a skimmer so ive been doing water changes every week or so and he was good. I dont know whats going on and im terrified of putting more fish in the tank. I dont know if it would be the conditions of the water. My damsels are doing great. Any help?

thanks
matt