First of all...the purple stuff is good, its coralline algae. Eventually it'll cover all your rocks and give your tank some color. As for the nitrite problem. Are you sure you do not mean nitrate, cause they are both way different. A nitrite problem would suggest that either you have added to many large fish too quickly or you tank was never fully cycled. It could also be that your biological filtration just can't handle the number of fish in your tank. Is the powder brown big? cause if he is, then he alone would probably be it for your bioload. I keep my tank at 79 degrees but there is many different opinions on what the temp. should be at, but somewhere around 78, 80 is good. You should invest in a heater just in case though. I'm not a disease expert by any means, but if your tank has high nitrites that could be adding to the problem with the tang.