If the GH out of your tap is 5 German degrees of hardness, that is already pretty good. Are the plants using the calcium and magnesium that fast, that you have to add more?
If the KH out of the tap is 2, and you keep topping off with tap water then you are indeed going to see 3, 4 and more degrees. This will help the pH to rise.
Try topping off with reverse osmosis or distilled water.