Death by algae??

[Borderjumper]

This hobby can be so frustrating! last night I cleaned the glass in my tank. Nothing out of the ordinary, just ran the mag float around the front. The only thing different is I also did one side, which did have a slight bit of cyano on it.

Walked back into the room about 10 minutes later to admire my handy work.. and bamm 2 dead firefish and 2 dead chromis with the other chromis flopping.
The little yellow tang and fat bastard six line are out swimming as usual.. WTF!