I know that you said the water parameters are perfect with the exception of kH but could you post them please?
I don't think that this is a bacteria bloom. You don't rinse the filter media in tap water do you? I'm thinking that what you could be seeing is an algae bloom. Water changes won't fix this. Algae blooms usually have a greenish tint to them, kind of like pea soup if it's really bad, but they can also be whitish in color. These are brought about by an excess of nutrients in the water-phosphates, nitrates or both. Lighting and especially sunlight will make the bloom worse. Blacking out the tank for a few days will fix the problem.
Here's how to do a blackout:
1. Feed the fish.
2. Once the fish are done feeding do a very thorough gravel vac and a large water change. Up to 50% is fine.
3. When the gravel vac and water change is done, turn the tank lights off. Cover the tank with a blanket or trash bags. Make sure that absolutely no light can enter the tank.
4. Leave the tank covered for three days. No peeking!
5. On the fourth day, remove the covering from the tank. If the water is clear, do another large water change and thorough gravel vac and the tank should be good to go. Problem solved.
6. If the tank is still cloudy after the first four days then continue the blackout for another 3-4 days or until the water is clear.
The plants will be fine and fish are able to go a few days without eating. I had to do this last year with a really bad algae bloom and it worked like a charm.
HTH,
Bryan