just throw a bag of activated reef carbon into your filter. This will clear the problem up within a few hours. Then, when it is done, you can either take the carbon out, or you can leave it in for three weeks. After three weeks, it will start giving off nitrates. It must be changed at the most at three weeks. However, if you leave it in the tank for three weeks, it will also take some trace elements out of the water. To counteract this, get Kent's Essential Elements and you will be good to go.
What I would suggest, just leave it in until the problem is cleared up. Then, remove it. Every time you start to see the tank getting a little foggy, throw a bag of carbon into the filter and let it correct your problem.
If you are using tap water, that is where your problem is most likely coming from. Most people try to find another source of water, such as the above-mentioned, RO. RO is just water that is totally purified and most fish stores sell it for 50 cents per gallon. I highly recommend it!