Joeri, in a fairly large tank, I find it easy to swish my river sand over to where it's supposed to be -- and it generally stays there as the individual grains, though quite small, are relatively heavy.
By the way, you might want to give your pleuros an artificial island or lily pad to climb up on to "bask" (for lack of a better word
) once in a while. Mine spend only about half the time in the water when given a choice. They rest all tangled up together.
s1ren, I keep my 10 or so P.waltl (4 adults and their mid-sized offspring) in a 90x45x45cm tank in which only half is covered by river sand. I drop the food (bloodworm, tubifex) into the bare-bottomed half of the tank.
If you mean "swallowing" sand into the stomach, I would guess that -- in small amounts -- it's small enough to pass through the digestive system without causing damage (though I myself still have some concerns on this score).
I'd also like to hear opinions on this from more knowledgeable folk
