Thanks, guys! There is actually very little potting soil in there--I was worried about it getting nasty, so there is probably only a total of a handful (maybe two) in there.
These fish are great in the tank right now: they move almost nonstop, occasionally resting on the bottom or the large rock...but then taking off and scrounging for food. Right now they are all 1" or a bit smaller, but full grown they will all reach about 6-9" (I'll release them as they get larger, of course).
"I go right into the aquascaping without any design."