Hmm. Thanks for the advice, folks. It does seem to me that settling sand would be a problem. Give that this is my first planted tank, I expect to do a lot of planting, moving things around, and generally messing with stuff. Maybe I'll start with just SafTSorb and save the sand for later. The STS doesn't look bad in it's own, but I thing the mix would bother me.
The tank is a 75 gallon Sea Clear System 2, with the built-in sump in back. I got the aquarium from Craigslist and am trying to set the whole thing up on the cheap. Bought some 24" T5 grow lights at a yard sale and plan to use decorative rocks and wood from the back yard.
I would guess that pool filter sand on its own doesn't make a good substrate for plants. Is that right?