If it is whitish cloudy, it likely represents a bacterial bloom. Greenish cloudy would be an algae bloom.
A bacterial bloom can occur for many reasons. One possibility is a new tank which is cycling (which your's probably isn't since it is 5 months old) and has rapidly growing bacteria. Another common cause is if there was a change to the tank that removed a lot of bacteria (e.g changing a lot of gravel at one time) the remaining bacteria will undergo rapid growth to fill in the newly vacant areas. This could also be true if you change too much water at once. The same holds true if for some reason there was a large bacteria die-off, perhaps from use of medication.
In any case, bacterial blooms, are generally not dangerous. They're just unsitely.
They should only last a few days.
Indirectly it may be dangerous, if it is due to the tank re-cycling, which can happen, but you said that your ammonia and nitrites are ok.
You can try reducing the amount of food too, as too much will promote bacterial growth.
Oh, by the way, if it is a bacterial bloom and you keep changing the water, you may just perpetuate it because you are removing the new bacteria and more have to form to replace what you have removed
If its an algae bloom (green water), I don't have too much experience with them, but reducing food and light as well as some water changes will help. Addition of live plants will outcompete the algae too.
Hope this helps,
lsmall
