Test kits are one of the most valuable pieces of equipment for anyone new to fishkeeping or setting up a new aquarium from scratch. The most important test kits are for ammonia and nitrite because both of those are toxic to fish. The next two in line are pH and nitrate: pH can affect fish and toxicity of ammonia, while nitrate can also be harmful in higher quantities to some species. GH and KH are also nice to know as they can indicate potential problems for fish breeding, development and growth. KH is also an indicator of how stable the pH is likely to remain in the aquarium as KH acts as a buffer. There are a few other things which can be tested for, but most of those are not of much interest in freshwater aquaria.
Test strips are not particularly accurate, so most people will recommend the use of “liquid” test kits (which includes reagents in liquid and solid form, which are added to a sample of aquarium water. The other thing to watch out for is that some test kits will only work in marine water or only in freshwater.
Although I have heard of some API tests being unreliable or difficult to read, I decided to try them anyway, mainly because of the price, but also to see how well they work for me. According to API (also known as Mars Fishcare Inc.), the freshwater master kit lasts for approximately 250 pH tests, 160 high range pH tests, 130 ammonia tests, 180 nitrite tests and 90 nitrate tests. I also acquired a GH and KH kit, but do not have an estimated number of tests for these ones.