Carbon filters strip the chlorine that makes the fishies float. Again, high flow is your enemy. Carbon filters work by adsorption and do not work at high flow rates. Have you checked your water chemistry before? Check the pH right out of the tap and again several hours later and see if you get a change.
If you are having trouble with the water right out of the tap, consider putting an aging tank for the waterchange water under the tank or in your laundry room, if it fits.