Once a tank is cycled proper and up and running, test kits are used mostly in case of emergency such as trying to find a cause of death etc. I Maybe test once every month. I have a few tests for plants, then those for fish. Such as phosphates, kh, gh, ph, nitrates, nitrites, ammonia. The api liquid test kit is the most widely used.
If you're just setting up a tank for the first time, and plan on keeping fish in it, you want to test daily to see how the cycle is going and make sure your parameters are safe for fish and if you need water changes. If cycling without fish, you just test every few days to see how the cycle is coming along.
55 gal heavy planted low tech south American tank
38 gal bow front live bearer tank
20 gal Marineland led brackish puffer tank
14 gal cf newly planted
10 gal Marineland led low tech low light
10 gal Marineland led qt tank
6 gal snail breeder
5 gal fluval chi dust collector
3 gal beta tank