Well winterose, they are helpful in algae cleanup and leftover food. They need a ph of 7 + for healthy shells and a calcium supplement to keep their shells healthy. They eat veggies such as carrots, cabbage, lettuce, squash, greenbeans (and all these veggies would be a calcium supplement). It depends on the angel temperment but my angel don't mess with them, mine are housed with an angel.
Promise they don't reproduce like crazy, like I said their eggs are above the water line in pink clutches, usually very visable. If you don't want babes just take down clutch by either pushing it in the water or by removing and sticking it in the freezer to make sure they are dead. A reasonable 2.5 gallons per snail b/c they get about the size of a golf ball. There are more threads here on AC that could give you more food types they will eat like snail jello that can be made at home. These snails are full of personality and too cool to set and watch clean and parasnail sail through the water. LOL