Aquarium Glass Cleaning?

i got 10 gallon Aquarium with one pair of Molly one pair platy and on Guppy pair with some fry i change my 80% water every week (on fridays) and i use cotton clothe to clean the glass also but after every 4th day the glass looks dirty again but water looks clean that is the reason i am chaning water every Friday is this normal how can i clean my glass or do i have to bring Alage eating fish or something. pls help