Sick fish. Help!

[madryno]

Help! Lost a couple fish in the last week. All was fine, then it seemed like they would start picking on a mate and the next day fish was found dead on bottom. Lost bannerfish to this issue a couple days ago, now two spot tang is laying on ground. No ammonia, nitrites good. We have had a nitrate spike we are dealing with, have done 20% water changes (2) in the last 4 days, nitrates still high, we were probably sucked into over feeding. No signs of ick or velvet on last two fish. I have attached photo of struggling two spot...thoughts?

 

[madryno]

Help! Lost a couple fish in the last week. All was fine, then it seemed like they would start picking on a mate and the next day fish was found dead on bottom. Lost bannerfish to this issue a couple days ago, now two spot tang is laying on ground. No ammonia, nitrites good. We have had a nitrate spike we are dealing with, have done 20% water changes (2) in the last 4 days, nitrates still high, we were probably sucked into over feeding. No signs of ick or velvet on last two fish. I have attached photo of struggling two spot...thoughts?