For cycling questions, this may be able to help
.. if not, then I would post in the habitat section
of the forums for cycling and pH questions..
But it sounds as if you have a hard water issue. Only thing that concerns me is you creating possible deadly fluctuations in the pH when you mix the spring with tap water- if not measured very accurately you can cause too much of a drop/raise when you add it to your tank.
What do you mean a 2 week change of the filter? Only the carbon part of the filter should be changed once a month.. sponge you will want to keep in there until it starts to fall apart- and when that happens, place in a new aq sponge next to the old one for a week or two to establish a new bacteria colony on the new sponge and then toss out the old.
After the tank is cycled, it will depend on how many live plants (if any) you have for water changes.. if no live plants, the tank is cycled, you would be looking at 40-50% per week with substrate vacuuming each time.
If planted, it will depend on how many plants, and of what type you have that will determine how much of a water change you will be doing.