It is better and more accurate if you have an API master test kit so that you can test the water yourself. Especially if your tank is cycling, you need to know what your water is doing so that you can correct it.
How long has your tank been set up? Did you cycle it first, or has it been set up long enough to cycle?
As far as food, snails eat zucchini, cucumbers, romaine, spinach, peas and sometimes sliced carrots. Haven't had much luck getting mine to eat fruit. Also, calcium containing veggies such as collards, kale and turnip greens are good. They also really enjoy algae wafers. It is amusing to watch them curl around the wafers.
Tums have too much sugar but you are on the right track. Supplemental calcium in the form of cuttlebone, liquid calcium and "people" calcium pills (without additives such as vitamin D) are good as well. Cuttlebone dissolves in the water which is absorbed by the snail's shells and I've been told that they eat it but I've never seen mine do that, and they absorb the liquid calicum too. They will actually eat the calcium pills but if they don't finish it, it could could your water, so be sure to clean the gravel afterwards.
As long as your water was healthy, you should only need to do a pwc. But you wont know that without a test kit. When a store says the water is "good", it doesn't really mean anything. You need the actual parameters.
Particularly if your tank is cycled/cycling you don't want/need to "purify" it. You'll lose whatever bacteria you might have starting to grow in there, especially in your filter. If the filter media is dirty, just rinse your it in tank water removed from the tank while doing your pwc.
