Thanks for the information!
I have both gh/kh+ and just regular gh+, so I have some room to choose. That's interesting though -- do you think the substrate absorbing the KH would occur quickly enough to cause pH swings sudden enough for a negative effect?
These two tanks are actually in a small room with no windows, so no sunlight! In fact, the room in general gets very little light throughout the day outside the aquarium lights (on a 24 hour schedule (finnex planted)). Not to mention, both tanks are in the same exact situation, and only 1 is exhibiting algae issues. Also, I plan on leaving most of it there (for now!) -- I'll only clean the front glass panel because ... well.. I want to see my shrimp