If its a well established tank then for a 240 with that stocking level id say just check the aquarium’s pH once a month, more often if its a relatively new set up.
Ideally test for nitrite levels weekly using a quick liquid or tablet test kit to pick up on any issues asap. Ideally, your nitrite level should be zero or as close to zero as possible. Even low nitrite levels (around 0.25 mg/l) can stress sensitive species, so anything above 0.1 mg/l should be considered unacceptable.
Also test for nitrate levels weekly. Compare the nitrate levels in your tank water with those in your tap water. Proper maintenance should result in similar nitrate levels between the two. Generally, aim to keep nitrate at 50 ppm or below. If your nitrate test reads 75 ppm or 100 ppm, you should perform a water change
