Murky water in biocube!!!!!!!!!!!!?

alright well i own a 14 gallon oceanic biocube, that is being cycled with a clownfish for about 3 weeks, about three days ago i took out the filter floss cartridge and replaced it with a protein skimmer made just for nano tanks, yesterday when i went to bed the water was perfectly clear, but when i woke up i noticed that it was a little bit murky!!!!! why is this!!!!!!!what can i do to make it clear again!!!!!!!!