do you mean cloudy when you say foggy?, cloudy water is due to one of two things, a bacteria bloom or poorly rinsed substrate. Both of which should have cleared up way before a month, unless with the former, you have severly overstocked. i know you said you rinsed 20 times, but sand can be very very "dirty" for lack of a better word, powdery for a better word, have you tried water changes, the amount of water changes that should be done in a month, especially with cloudy water, should have take care of the problem.
And yes water polisher filters are great, but that doesnt solve your problem, you need to figure out WHY the water is cloudy/foggy and the remedy that.