What I do is shove a piece of sponge from an AC110 into the end of the gravel vac tube when I refill the tank. This gives something for the microbubbles to attach to and they come together to form larger bubbles which easily dissapate.
I always just figured it was kicking up sediment when you dump water back in the tank. Except, I just vacuumed the gravel, so there shouldn't be too much sediment. Maybe tap water is just coudy and it doesn't clear up until it passes over the charcoal in the filter.