I actually use Hikari algae wafers and then for the extra calcium I feed mine unflavored tums, they love them and a vet friend of mine says that it will do them absolutely no harm. Add some crushed coral to the tanks and the pond that should help stabilize the water parameters. Also I don't know if it is the best idea to put snails that you buy into a pond with koi. I kept some of my larger snails, half the size of a baseball in a large tank with a shubunkin and the koi would just suck them right out of the shell. Talk about heated.