Only fish people talk in degrees hardness which is "German speak". Christchurch water is about 45ppm hardness expressed as calcium carbonate which is about 2.2 deg. so at 5-6 deg (110PPM) your hardness has increased quite a bit. Christchurch water has a pH of about 7.2-7.4 so it has dropped quite a bit. Your goldfish are heavy feeders and heavy polluters and your filtration will generally take out the lumps and harbour bacteria usefull in the nitrogen cycle. It generally will not alter pH or hardness and these changes are arising in my view from the load on the water from feeding and excrement. Change the water and you will change these two factors or spend a fortune on chemicals and keep the fish shops and yourself very busy. One of my tanks has 3 large chunks of marble in it and I doubt that it increases the hardness by much. There are many forms of calcium carbonate and they are all but insoluble except in acid conditions. If you change the water you will not need to buffer it and you will raise the pH again. 6.2 is not very acid--- Coca Cola is about 4.6 if remember correctly.