Get trests to follow the Nitrogen cycle....
Ammonia, Nitrite & Nitrate plus you'll want to test pH also.
It's nice to have the hardness (GH & KH) test too, though it's not critical.
Ammonia and Nitrite are deadly to your fish, Nitrate is also, but only at high levels, but this nNitrates in the water will tell you when to do your water change as bacteria convert Ammonia to Nitrite and then Nitrate, but Nitrates are removed with H20 changes. (plants can use some but not all)