You could use plain potting soil with NO additives. Place it in the bottom of the tank about an inch deep and then cover it with fine gravel about two inches. Large gravel wouild allow too much of it to leach back up through and dirty up the water. Many folks use this for natural aquariums. you can google info on natural aquariums for more info. when filling the aquarium put a dish or saucer in the bottom and pour the water slowly onto this so that water doesn't cloud too much. I'ts gonna cloud up but with filter running for a day or two and if poured slowly as described it shouldn't be too bad. It will settle. You will as you already know, need to be careful while vaccuming. Once you put the potting soil in that's the time to plant the tank BEFORE you add the gravel. this too will keep clouding or muddy water to a minimum. hope some of this helps!