Stabilizer shouldn't be going up unless you're adding it--there's usually no stabilizer in fill water, so I don't know where it would be coming from. The only guess I have would be from testing. the CYA test is very subjective, and if the lighting conditions aren't the same each time you test, the results can be very inconsistent. For example, if the light is dim, you're going to see the dot disappear quicker than you will if you're testing outside in bright sunlight. The key there is to just try to keep your testing conditions consistent from one time to the next.