Apple snails shouldnt eat plants unless there's not enough food for them. Give them algae wagers, cucumber etc.
Snails don't eat fish waste. They just eat left over food and algae.
I'd have thought apple snails would like hard water, the calcium in it would be good for their shells ?
I know acidic water is bad for them.
Also, whilst they do eat left over food etc, they can leave a fair bit of waste themselves and can be a bit messy.
They do have a good character though. Coming up the top for air then letting go of the side and just crash landing into whatever is below
Be careful with what type of snail you get though. Those that you get free with plants - usually pond snails or something similar, do breed very rapidly and if not careful can soon take over the tank.
Also, it depends what fish you have. Loaches are known snail eaters.