Susan, there's actually not much that
absorbs water less than perlite, and you should not be afraid to use it.
Perlite does not absorb water, but it does hold it in irregularities of the
outer surface. Vermiculite
holds water in the
multi-layered body of the granule, MUCH more than perlite. I suspect
that you might not have drained as much water out of that tub as you thought,
and the water in the bottom kept wicking slowly
to the top, keeping it damp enough
to grow that algae.
Only thing I can think of better than
perlite to open a mix up is pumice, which is only available in quantity on the
far west coast. Smooth round styrofoam beads, like I see in some
commercial mixes, might be better than perlite if one could find it,
but I've never seen it.
>>>....perlite never did dry, it did develop a nice green
mold .... I've been leery of mix recipes that call for a lot of perlite to keep
the mixture from becoming too moist. SusanB