Sorry to resurrect this thread yet again, but I just found it.
I have never used Excel as an algicide or otherwise, but I have used hydrogen peroxide (H2O2).
It seems like Excel and H2O2 have a lot in common. Both act as broad-spectrum biocides. Both seem to work their way up the evolutionary ladder with increasing doses - algae, mosses/riccia and delicate plants, shrimp, most plants, most fish, everything.
With H2O2, it's common practice to squirt it directly on patches of algae with a syringe to maximize its effect there, while minimizing the impact on the rest of the tank.
So here's my stupid question. Has anyone tried squirting Excel directly on algae?