Hi,
It is likely to just be an algae bloom and is common in new tanks and will usually clear up of its own accord.
Not sure how long you have lights on the tank but dont have them on any longer than about 6 hours a day for the min if you need them on at all, if the water doesnt clear up you can do as you mentioned, a blackout which will help kill all the algae.
Andy
Thanks,
We've not really put the tank light on as the room itself is quite bright, had it on yesterday for a few hours yesterday and early this morning, but won't be doing that again for now. We have done a partial water change, cleaned and adjusted the filter so i'll see how it goes over the next few days.
dont clean the filter unless the flow of water has actually being slowed by sludge or your just getting rid of the bacteria youve already built up in there, if you need to clean any sludge build up then make sure you only give it a quick rinse with the dirty water from the tank itself and NOT tap water. never clean it properly just enough to free up the flow again