Hmmm,
well I thought not only from the substrate u put in the water which can make it cloudy - but when u add, water conditioner - this gives the bacteria in the water and such to thrive for a bit as the water conditioner removes the chemicals which inhibit amoeba/bacteria growth.
It takes a while for it to clear until the filter in the tank is getting properly established and a few water changes have been done - we ran the filter for our second tank in our other tank to get the bacteria growth and cycling going - before we even put it into the new tank - and we had like 1 maybe 2 days where it was cloudy ,now its just normal.