Water tests, what are the important ones?

Testing

For awhile, you want to keep testing pH and KH and all the other things too. When you are certain that the ammonia and nitrite are always 0, you can stop testing that for the most part, but whenever things look odd, test everything again.

What you are looking for is a sudden change in conditions. Some people report that their water department changes sources several times a year, from lake to wells, or rainwater or snow runnoff dramatically changes the river water. This can uspet the fish, so it is good to know in advance so you can reduce the size of changes and increase the frequency at those times.

Also, certain medications can trash the filter, so you'll want to know ammonia and nitrites then. It is good to test that occasionally, just becasue the overstocked tank gets worse as the fish grow up.

High nitrates can be your first clue to poor plant uptake in the planted tank. It probably means that you are out of something else, CO2 is low or phosphates or something.

For the first 4 months, I tested everything weekly. Then I tested nitrates, pH, KH, phosphates weekly, later I slacked off to monthly. Pretty soon I forgot about testing completely, until I notice something not right. Now, I'm pretty lazy about it, too many tanks and the juvenile discus take so much attention with feeding and water changes that testing is pretty low after pruning and water changes the planted tanks.