How can i make my Aquarium Water Clear?

Hi everyone i have a Juwel Rio 240 litre aquarium and its been set up for around 2 months now and has around 20 fish in it, i am having problems with the water i cant get it tobe crystal clear or even just clear, it is cloudy and i dont know why i have done several water changes around 30/50% and it doesnt make much of a difference so i wondered if anyone can give me some tips on how to get it crystal clear like my other 2 tanks are as i,m puzzled many thanks?