possibilities: 1)not enough filtration; 2) bacteria bloom; 3) washing your filter media at tap water; & 4) algae in the inner glass wall.
what type of filter system do you have?
when you do water change, did you clean the filtering media?
did you clean the inner wall of your glass tank?
the slimy materials in the filtering media actually help to keep your water clear. however, we always thinks it should be washed and cleaned. in doing so, particularly in washing from tap water, we kill the good bacteria resulting into cloudy water.
another thing is the algae formation at the inner wall of the glass tank. please check.