Welcome to the forum Murph123. I am sort of with Webber on this one. If you lose fish each time you do a water change, there is probably something wrong with your technique. A water change is usually greeted by my fish as a time to celebrate, not a time to die. It is important to roughly temperature match the new water to the old tank water and is also a must to treat the new water with a good quality dechlorinator. I use Prime but there are lots of decent dechlorinators out there.
As JoshJames said, the filter really only needs cleaning when the flow starts to be restricted. For a hang on back filter that could be as soon as a month while for a canister it could be as long as 6 months. It is something you will need to judge by observing the flow in your tank.
The chemical testing will help us understand where your tank is in terms of the nitrogen cycle. Only the liquid reagent types are generally trusted to give reasonable readings so don't be tempted by the slightly lower price of a test strip. In the long run, the test strips inaccurate readings lead to wrong actions and are best not being used at all.