I've also noticed in my seahorse tank after doing a water change, a day or two later the water becomes "milky" looking for a couple of days, then crystal clear until the carbon is replaced again. It's been suggested to me to be sure to rinse the carbon inserts before putting them into the filter. I haven't replaced the inserts since, but will be soon. I'm hoping it's going to work. Could this be happening after replacing your carbon?
Lisa