Hydrometers are not as bad as everyone says they are. They may not be accurate, but they do not flucuate. Take your hydrometer to your LFS and compare it to their refractometer or anyone you know who has one . Both my hydrometers register 1.026 but on my refractometer it is 1.024. So I know that my hydrometers are off by .002. Compare yours and then you will know how much if at all it is off.
Anyway, I would drain some water in yor tank and add fresh water until your tank settles to where you want it. It could be that you are just replacing the water that is evaporating and not really dilluting the water at all.