As a rule I check PH after every water change.. I donhave API's kit for liquid testing but I found for PH this faster and just as reliable and less mess..
KH & GH as well during water change...
But one thing about hardness is once you start messing with it, its a life long process. Every water change will effect hardness...
KH & GH also effects the Buffering capability of your water...
Mother Nature loves Balance, so you can spend money Buying PH up or PH Down or Set PH 7 and you will get it to go there for a period of time but it will slowly return to What it Came out of your Faucet at because of Hardness...
Mind you in theory those chemical additives sound all fine and dandy but they can and will cause Moderate to High PH Swings. And a swing over .2 in PH ina short period of time can cause fish to die...
A couple ways to Manage GH in a Controlled fashion is:
Ways to reduce gH
Adding peat moss to your filter
Use commercially available water softening pillows or a water softener (this removes calcium and magnesium ions and replaces them with sodium ions. Many people feels that this is an unacceptable method of softening water as many fish that prefer soft water don’t like sodium either.
Mixing tap water with reverse osmosis (RO) water.
Ways to increase gH
Adding limestone to the aquarium (this will also increase kH which in turn will increase pH)
Adding calcium carbonate will raise gH and kH
I also found that these work well with WATER CHANGES to stabilize the tank in the first day
I hope this isnt to much info, im usually on Planted tank forums where detail is a must... Hope this helps