It worked!!!! Almost all of my dark brown algae is gone and now my plants are starting to grow fast and bright green again. I did one treatment of 9 tbsp for th 72 gallons without fish and then 10 tbsp with the discus and cardinals and plecos. Low and behold no deaths! Untile a couple of days later and then one cardinal vanished?! Not sure if the H2O2 killed him off or the discus wanted a midnight snack. Anyway, thanks a bunch for turning my tank around!