Answer:
In most fish tanks the water is pumped through filters to control turbidity.
Cloudyness (turbidity) of the water is due to suspended particles in the column. The particles can be bacteria, suspended solids or gaseous air bubbles. Simple osmosis will eventually dissipate the latter. Mechanical filtration will remove the suspended solids that do not sink to the bottom and a combination of filtration and water changes will remove the first named cause (bacteria).