Hmmmm. (scratching head)It could be a bacteria bloom you keep in bloom with large aggressive water changes....maybe? How many fish? Is there major circulation in this tank? Possible stirring of the substrate? Define "milky"....sorta hazy like fine bubbles in the water colomn, or milky like there is a portion of diluted milk in there?
Do you have a test kit to test for ammonia, nitrite and ph? If this tank is lightly stocked and is still cycling then maybe that would explain the persistant cloudyness.
Hold off on waterchanges for a few days (providing you keep an eye on your ammonia and nitrite levels) and see if the cloudy deminishes. Watch your levels and if you see rapid increase in ammonia over just a few days, then do a water change. I would do a 20-25%.
Overfeeding frequently causes bacteria blooms due to the over abundance of nutrients in the water.
A U.V. sterilizer could be a solution.
Good luck.