There are many different types of test kits commercially available. If you are concerned about water balance, (and you should be) you will want to spend more on a nice kit. The basic “duo” test kits, available for about ten bucks, are usually OTO chlorine and pH testers only. You may wish to spend $10 more for a DPD chlorine kit, which measures free, combined and total chlorine levels (OTO measures only free levels).