If you don't mind spending the money on a tester you will use only once, yes the API GH and KH kit will give the information you need. Unless you do something in the tank to alter hardness, it won't change which is why you only need to test once.
Just so you know what to expect, these testers work differently from our others. With GH and KH, you add 1 drop at a time and shake after each drop. The end point is when the water changes colour. Make sure you count the drops - with the API testers, the number of drops is the same as the hardness as dH. As your water is on the hard side it will take several drops (somewhere between 10 and 20) so the colour will be quite strong by the end point making the change easier to see.
Just so you know what to expect, these testers work differently from our others. With GH and KH, you add 1 drop at a time and shake after each drop. The end point is when the water changes colour. Make sure you count the drops - with the API testers, the number of drops is the same as the hardness as dH. As your water is on the hard side it will take several drops (somewhere between 10 and 20) so the colour will be quite strong by the end point making the change easier to see.
