When do you do water tests?

at the very least do one before and after a water change. The one before lets you know if you are changing your water out frequently enough, the one after lets you know if you changed out enough water. Test every day if you see fit, you can never test enough.

I test every other day since my aquarium is new and has not completely cycled yet. When I finally get some results that are decent, I will test once a week. Once the aquarium is "established", frequent test shouldn't be necessary.....but I will still do them.

I don’t think it really matters (before/mid/after) as long as you are consistent.

For nitrates, for a while, I would test before and after, as Lauren suggests, to determine the right amount of water changes for my tanks. Take notes! Now, I only test every week before a water change to see if my nitrate levels are still what they used to be for each tank. If markedly different, I adjust my water change amount and do the before/after for a while again.

New tanks require frequent testing, but since my tanks are established, I only test for ammonia and nitrite monthly – or after a vacation or fish addition.