I do maintenance on a number of tanks and i have had this happen to me a couple of times now. it seems that when i scrub some algae that has been growing for awhile like on the back of a tank fish seem to be lacking oxogen. they start breathing rappidly then start swimming funny and end up on the tank bottom dying within minutes. i have lost firefish, tangs, gobies, chromis, is this possible nothing else was added or changed can scrubbing certain algaes deplete o2 levels that quickly