I'm sure this information is elsewhere on the board, but I was unable to find it quickly (which probably means others can't find it quickly), and due to an increasing trend I see in questions I get both personally and that are posted on the board with people not having critical test kits, I want to put this information up at the top in direct form for a while at least. Each of the statements/questions I've listed here I've seen over and over again.
What test kits do I need to keep apple snails?
If you're going to keep any aquatic animal (fish too!), at a bare minimum you need test kits that can handle the following:
These test kits are expensive! Are they really needed?
Yes, they can be expensive, but YES you need them! If you are going to be a responsible pet owner for any pet you put into that aquarium, you need to consider those test kits as one of the mandetory expenses for setting up an aquarium - just like the tank itself and the filter. You should also have these test kits long before you buy any animals if you want to avoid common problems that could result in you loosing all of your new pets.
The pet store I went to said I didn't need test kits.
Although pet stores often try to sell you everything they can, I have seen this rare event. The answer is simple: they're wrong and are either very misinformed or are just trying not to loose a customer because they know most people want to cut costs and buy animals instead. Pet stores, especially large chains, give out LOTS of bad advice. Many people who work in the fish/aquraium departments of these places are not experienced hobbiests themselves. Stores run by long-time aquarists will recommend exactly the same thing as is stated here: get the test kits if you want an aquarium.
My pet store tests my water for free. Why do I need to own test kits?
The first three test for toxic chemicals that kill aquatic animals. Read up on tank cycling to understand why this is important when you first set up your tank. Even when a tank is cycled and stable, there are common things that can go wrong and produce spikes in the concentration of those chemicals that are hazardous to your animals. If you can't test for them, you might not identify a problem fast enough and your animals can die quickly. Some stores will test your water for free - but what if you start seeing your aquatic animals act sickly all of a sudden (or even start dying) when the store is closed? This is why you need to own the kits, not just take water samples to the store.
Are there any other test kits I should have?
For snails specifically, kH. The kH test kit is not as critical to aquarium keeping as the other four listed, but if you're going to keep snails, you should also have a kH test kit. This test kit need not be used as often as the others but is a useful diagnostic especially when shell problems are involved. If your pet store tests for kH, you might be able to get away with just taking water samples to the store for this particular test. If there are persistent problems related to pH though, you really should have a kH kit handy and not just use the store.