why do some tanks do not have green stuff on the gravel and some do?

[KAMALDEEPSAINI]

i have a 20 gallon tank and every week i clean my tank there is so much green stuff on the gravel and i checked my freind's tank and his gravel was so clean?

 

[loupa_carpathian]

You may have more algae production than him, or he may have a better/more/larger sucker fish.