POWDER BLUE TANG ICH and yellow tang aggitated

I have I have a yellow tang and a powder blue tang in my 70 gallon tank . Everything was fine but yesterday I noticed that the yellow tang was like aggitated and started swim ming fast and even glass surfing . Then today i notice my powder blue tang all of a sudden it has white sports on it. I tested the water and my nitrate is 50 but it's been 3 weeks like this and I can't decrease it with nothing . What can I do to help the fish pls help