I'd recommend having testing supplies for both nitrites and nitrates. Nitrites are what you really want to worry about, but high levels of nitrates isn't a good thing either. You may just be able to buy a test kit that has various tests included, like I did, or you can go out and buy seperates. In either case, it's important to be able to test your water parameters since that is the first thing you're going to want to know if your fish are sick.