Best one from 7 tested: Marine Depot-American Marine Pinpoint pH Monitor Package
If you decide to go cheap $7-10 buy 3-4 in order to compare readings. I like them better than Hanna and Milwaukee ones. Buy 4-7-10 PH fluids to calibrate them. Cleaning solution and storage solution if you won't keep them all the time in the water. Buy some API PH test just in case. All those steps are considered only if you have expensive shrimps. Other ways API are good enough.