That's weird! I've lost fish 3 times after cleaning the sides of a tank. Just basic mag float scraping... Nothing on the mag float, nothing on my hands.
As you said within MINUTES the fish are stunned, lay on the bottom and are dead almost immediately. The funny thing is it killed the smaller fish almost instantly while a few bigger ones swim around like nothing's happening.
Victoria Killer Algae maybe?
I now clean the glass on my tanks a 1/4 a day I'm so freaked!