If the cloudy water follows the cycle from dark tank to lighted tank and back - it could be due to a few things.
To try and narrow it down - may I ask a couple of questions about your tank.
Do you dose any additives or drip kalkwasser at night ?
Do you have a relative heavy bioload of fish compared to the size of the tank ?
What do you have in way of internal tankwater circulation ?
Do you have a lot of green algae growth ?
The cloudy water could be a result of Ph swings - affecting alkalinity / calcium balance.
The pH could be swinging due to CO2 gas, algae, fish consumption/production of this soluble gas
Or
Dosing an additive
Or
A bacteria respose to changing water conditions from day to night.
Have you tested the pH, alkalinity and calcium lately ?
If so, what readings ?
Does your pH vary from day to night ?
Lastly - do you have any nocturnal sand dwellers/fish/inverts that may be stirring things up a bit down there at night ?
( I just realized I'm in the disease/treatament forum
)