1-month tank, cloudy?

I have a tall 15g (15H) tank with an under-gravel filter. It's been
running a little more than a month. I put a couple of feeder goldfish in
it a couple of weeks ago. Things have been fine. I've done weekly
partial water changes and have scraped the glass a few times. The pH has
been steady about 7.4. I removed the comets and put a couple of mini
fancy goldfish in about 4 days ago (Siamese Dolls). The water still
tests fine, but it has gotten cloudy, like milky. When I do water
changes, I rake over the gravel/river-rock and vacuum it. Been feeding
once a day, pellets soaked in aquarium water. I haven't changed to
carbon cartridge yet. Any help appreciated.