Per Seachem's own instructions (and some help staff interactions), Excel is not meant to be algaecidal. That's an off-label use that we in the amateur scene have picked up. I don't really recommend it to be used solely as an algaecide in a holding tank, especially if it's not planted. As always, you're better off figuring out what's causing it, which shouldn't be to hard in that case (hint:it's your light).
Bad guess This tank is a temporary plant quarantine (snail death chamber) and it won't be up long enough to balance. My goal was to push growth on some new plants while keeping the algae within semi-acceptable standards until the plants are dipped and moved to the main tank.