Sorry to hear about your SHs, Neal
I suspect it may have been the algae you scraped off the back of the tank. Was any of the algae red and stuck on like coralline, but not coralline? I had some of that stuff on the back of our 120g last summer. I didn't know what it was, but it was overtaking the coralline, so I scraped some of it off. Was away from the tank for 3-4 hours only to come back to two dead fish - my favourite ones! - and the others gasping for breath. Did a 40g water change and threw in some carbon right away, which saved the remaining fish.
What was the red algae on the back of the tank? Don't really know, but I'm guessing it was a species of cyano that, when disturbed, released a cr*pload of toxins. Corals were not affected.