If it's more of a white color, it's a bacterial bloom. Nothing to be concerned with as long as ammonia and nitrite are remaining at 0ppm. It happens sometimes for various or unknown reasons and goes away on it's own, sooner or later. Water changes never seem to help it, IME.
If it starts to turn green, cease and desist the water changes and airstone and start looking for a cheap UV sterilizer. Recent changes in the fert regime point toward that possibility in a big way.
Also, make sure you don't have any yeast mix making it's way into the tank, 4 bottles gives a much higher probability of that happening.
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