Wow, that's crazy... cause I've never added nutrients to the water column, and this tank has been established for many years (I'm only now getting into the plant thing). My substrate is plain old gravel. And the green water started after I added the light, but before I started with the substrate tabs.
(flourish excel isn't nutrient, is it?)
thanks for the info, though - gives me something to work with!
IME, too much light period also cause green water.