My guess is still off gassing of co2. The guys over on SimplyDiscus often have problems with this, especially in the winter time. They will do a water change and the water that they put into the tank from the tap will sometimes create a cloud of co2 bubbles. That's when they know that they need to age their water. Btw, dissolved co2 is usually a bigger problem in the winter because the normal tap water temp is lower. This causes more co2 to be dissolved in the water and then releases more when filling your tank with it. This is also part of the reason why high tech planted tanks usually have their co2 shut off with their lights. If they left it on with no light then it just accumulates in the water because the plants have to have light to use it. This causes the ph to swing a lot over night and then swing back the next day when the lights come back on and start using up the co2 again. Having said that, it is also bad because more dissolved co2 can displace dissolved oxygen and suffocate the fish. Both are problems if the co2 isn't shut off when the lights go out.