Could you give more information about the tank?
How many fish/what types
How often you feed
What you feed
A month is still a really new tank...sometimes it takes several months for a tank to fully cycle. The best bet is that it is still cycling, and is undergoing a bacteria bloom. The tank usually starts out crystal clear, and remains that way until an ammonia source is added. That's when the bacteria appear and make the water a milky color.
Just bare with it. Keep doing regular water changes...and test the water frequently. You want to have 0 ammonia, 0 nitrites, and nitrates under 40. When vacuuming the gravel, don't go too crazy, maybe only vacuum 1/4 of the gravel each time, or non at all at times (depending on the fish load). This will ensure that you don't destroy and kill too much of the bacteria living in your gravel.