Cloudy Tank

[keithm]

hi if this is in wrong forum am sorry, my question is a tank i have set up is clear as daylight with the light in tank not on, but soon as night time i switch it on, i can hardly see a thing,

 

[Gruntle]

If it's a new tank, you're probably dealing with a bacterial bloom. It happens to most, if not all, new tanks. Until your tank cycles (see the link on Cycling a Tank at the top of the forum) you won't have the right species of bacteria, and the ones that turn the water cloudy will remain. It will clear up, there's no need to spend money on a clarifying agent or anything (and if you use one, the bacteria will just multiply again and turn the tank cloudy).
 
Can you give more information, is it a new tank, do you have fish yet, do you have plants, are you attempting to cycle the tank?