scrubbing algae killing fish

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

i use the same pads in all my tanks nothing special just a white algae pad again its old algae mabey a little slime here and there i always make sure they are clean nobody touches this stuff but me

this is a 90 gal flatback hex with built in filter and skimmer in the back never had much problem with it coral all do well had a red slime problem dosed with a new product from blue coral for slime ran carbon and changed 20% slime is gone. last week i cleaned gravel line and kicked up sediment from gravel it killed a firefish and a yellow tang within minutes clown and scooter blenny survived as did all the coral. today i added a bar goby and a kole tang both looked fine then scrubed the back of the tank to remove excess algae and then both fish went into serious shock breathing hard and could not keep equilibrium dead in less than half hour. i know its not my pads as i use them all my tanks that i maintain and dont see these problems. tested calcium at 350 alk at 9.6 dkh im wondering if their might be some toxic state caused by dosing this red slime remover that lays dormant in the sand and or on the back of the tank until it gets stirred up. i had a similer problem in a 400 gal some time ago again it was after i scrubed of the back wall of some thick green algae with a little hair algae mixed in killed numerous fish same way looks like o2 deprevation all fish die with gills wide open seem to be gasping for air. again all coral seem fine i ordered a o2 test but thought coral would suffer as well with low o2 levels i am at a loss not to mention this tank is in a doctors office so everybody gets to watch this fish die :huh:

tank has been running for about 5 years it was moved about 3 months ago i use seachem's reef builder and calcium supplements water change every other week carbon is changed out every month use reef crystals salt seachem's phosguard when needed havent checked ammonia or nitrite but pretty sure its nonexistant. nitrate around 50ppm most my tanks run this high without a problem whatever is doing this seems to happen when either i scrub algae or turn over the sand it seems. i dont vacuum the sand but am considering doing so tank runs 80 salt at 1.020 have you heard of TOXIC algae???