Decide for good quality test sets from manufacturers specialised on salt water aquaristics! Due to they might not be standard items at your local dealer, it might be a good idea to purchase them in Aquaristic Online-Shops.
Unfortunately, there are also several test sets, even so called “profi-cases”, available that measure/display at least inaccurately or even totally wrong.
Well established manufacturers of testsets for reef tanks are:
Tip: Carefully read also the FAQ-point: “Hints to reference solutions for test sets”
Carefully read also the FAQ-point: Hints to reference solutions for test sets