You can buy one at some of the major discount stores, or a pool supply place for a few bucks. It will be very handy for you.
pH should not be below 6.8, or the water will not support the algae bloom necessary to support the aquatic invertebrates (bugs) needed to help support the fish.
If the pH is not high enough, you will need to use an agent, usually agricultural lime, to get it higher.
How much you will need to use, is dependent on the pH, and the amount of water. I will add information, and possibly tables soon.
