You can feed veggies for calcium - kale, cucumber, etc. or you can get cuttle bone which is found in the bird section, its the white thing birds chew on for the beaks I believe. I wouldn't worry about dropping an algae wafer for a week or two but the tank with 1 nerite I would only drop 1/4 or 1/2 a wafer. I will add, I have not seen my nerites eating an algae wafer but that's not to say they don't when I'm not looking. If you get the Mystery snails, then drop a whole wafer in each tank, they LOVE the wafers. I started with Mystery Snails & instantly fell in love with them.