Hello and thank you for your inquiry.
It is hard to say exactly why the tank gets cloudy, but more than likely it is due to distrupting the biological balance within the tank. If you have a sand bed in the tank you may be distrupting that as well causing some cloudiness within the the tank.
If the tank clears up within a day and you are not seeing any ill effects on the livestock I wouldn't worry too much about it, but you may consider doing more frequent smaller changes in the future.
If there is anything else we can do for you please let us know.
Marine Depot Customer Service