If it is milky white cloudy water then its a bacteria bloom this is not nice to look at but its a good sign in a cycling tank it just mains you are producing lots of good bacteria so much so that it is depleting the oxygen
A Bacterial Bloom is Part of the Nitrogen Cycle
Most of the bacteria in the aquarium are aerobic as it is a oxygen dominated environment, and these bacteria require lots of oxygen. When the heterotrophic bacteria bloom into the water column and switch to their aerobic state, this is a big drain on the oxygen content of the water.
Oxygen deprivation is the only risk to the fish during a bacterial bloom, as the heterotrophs themselves are harmless to fish, (fish may be gasping for air at the surface of the water during this part of the nitrogen cycle), and so good advice is to increase aeration! with a airstone or powerhead it should clear up after 12-48 hours if you have no fish in the tank then you have nothing to wory about but I would increase the surface agitation.