How can i get rid of cloudy water?

i have a 20 gallon tank, atificial plants, and no fish yet. i just set it up 3 days ago. it has a brand new filter. the water was fine up until yesterday, when i put a new submerged heater into the tank. then when i came home from work, the water was cloudy. today i did probably about a 40-50% water change, and cleaned the filter, and it is still cloudy. how can i clear it up?