Are you talking about the white fog? If so,IME, that means you have bacteria eating ammonia. If your API test kit is like mine it will appear to show small amounts of ammonia while actually it means 0. It's an oddity with that kit but you'll get used to it. If your sand stayed wet when changing you may have good bacteria on it.
Are you sure about that lol... My 5.5 betta tank has been setup for like 2 weeks now and it has been cloudy for almost all 2 weeks... The ammonia is at like 4.0 and wont go down... My nitrite is 0 and my nitrate is at 0-5ppm which is what the tap water has... So how can the cloudiness be bacteria eating ammonia when I have no nitrates and nitrites..