Killed my fish trying to kill algae

I had a major catastrophe in my tank today - 12 dead discus and assorted tankmates for no apparent reason. I would appreciate some help in figuring out why.

I have a 180 gallon tank, moderately to heavily planted, with all appropriate fertilizer levels and CO2. I added a new piece of driftwood to the tank 3 weeks ago, and it started to grow hair algae on the top branches that were close to the surface. This morning I used a turkey baster to inject some hydrogen peroxide directly onto the algae. Total peroxide used was 3-4 ounces of standard 3% solution. About 2 hours later, my fish started dying off one by one. An hour later I had lost 12 discus, around 10 cardinals and one blue ram. I am left with 10 more cardinals, 3 cories, 10 or so ottos and 2 gouramis all of which seem unaffected.

I don't understand what happened.

Two possibilities:
1. The peroxide was actually much stronger than 3% even though the bottle said 3 percent. It was a no-name brand that I picked up in my local supermarket.
2. I later read that peroxide interacts with ferrous iron to create some really nasty compunds that destroy all organic tissue. My iron level was around .3ppm and I had added more 2 hours before I put in the peroxide.

Either way, if the water became toxic enough to kill all the discus, why didn't it kill everything? Any thoughts?