Hi all,
Problem is that Planaria are much more active at night, and although some fish - Anabantoids and live-bearers (Trichopsis pumila, Betta splendens, Poecilia caucana & Badis/Dario spp.?) eat them and might be small enough to be suitable, they will only control the population when it gets large enough for some Planaria to be active during the day. When you can see some, you have loads you can't see.
I've tried getting rid of them with Flubenol (they are mainly harmless, and don't stop Plecs and cichlids breeding, but they eat unprotected fish eggs from Tetras, Cories etc.) and it works, but I've always got them back.
I now use baiting to reduce their numbers, this consists of a gauze bag with a prawn in it, tied to a thread and removed about 2 hours after lights out. This also works for leeches. Another collection method is with a small square of slate, it needs to be flattish, but not entirely smooth. The Planaria will cluster underneath it during the day, and can be removed.
I usually dip the bait bag (or slate) in some boiling water, as leeches particularly cling on very tenaciously and are difficult to wash off or siphon up.
cheers Darrel