well, do you have the fish in there yet? Your tank is looking very nice. Those sorts of snails will readily eat any decomposing plant matter, left over food, etc. If you haven't added your stock yet, you could drop in a piece (not bigger than a postage stamp) of cucumber, zucchini, carrot, green bean, lettuce, spinach.....anything really, just microwave it for about 15 seconds, or drop it in boiling water briefly and then use a clip to sink it in your tank. The blancing/microwaving helps break down the cellulose so its easier for the snails to digest, the same thing can be accomplished by leaving it in the tank for a couple of days. If uneaten after 1-2 days, remove and replace. Hope this helps. You can use a plant weight, chip clip, fork, etc to weigh down veggies.