Hello jo...
A rule I learned some time ago and has worked well for my tanks is: Tanks up to 30 gallons, need half the water changed once a week. Tanks larger than 30 gallons should have half the water changed every two weeks. The reason being that large tanks have more water in them and more water dilutes the wastes the fish produce, so the pollutants aren't harmful to the fish.
If you're a good water changer, then you don't need to test the tank water to make sure it's safe for the fish, because large, frequent water changes always keep the water safe.
B