If you want to add some more cap in the setup tank, put it in an (empty) soda/water bottle, fill it with water, and then put it in the tank, invert it a few inches (or more), and repeatedly squeeze the bottle.
The substrate will tumble out, and most of the dust/cloudiness will remain in the bottle. Take the bottle out, dump the water, and repeat over any thin/non-capped areas.
Someone used to post a youtube vid demonstrating this, but I can't remember who it was, or the video url. I've done it a couple times to help cover exposed roots or add substrate, and it worked pretty well for me.
As to the salt in the mushroom compost, that wasn't something I was aware of, but I would think that a few water changes would get most of it out (still, wouldn't want to put fish/plants in it until then.
Anyway you can do multiple buckets or shelter some of them in existing tanks (maybe throw in some separators), or anything?
I've got quite a few malaysian trumpet snails, and they stir the surface around a lot, but I don't think they go deep enough to disturb the soil layer. Little pebbles, small chunks of wood, and some bones scattered in the tank get moved all over the place, but I haven't really noticed any dirt surfacing. I don't think they will disturb the dirt layer unless your cap is exceptionally thin.
Last edited by lochaber; 09-25-2013 at 07:33 PM.
Reason: add snail info