Stop doing water changes.
You need to let the tank stabilize again. What kind of water are you using? (ok wet ;) where's it come from?). To stop the build up of hardness and chemicals from the tap water I use water from my dehumidifier, which is like distilled water but a lot cheaper to...
did you actually see the severed belt?
It happens, I change mine at 1/2 the recommended millage interval just in case. You might be in for more than just a head you might well have had the valves punch holes in the top of the cylinders and have the resulting debris score the cylinder bores...