What is a cause and cure of the algae in my tanks?

Okay first of all I have a 5 gallon tank with 1 male betta in it. It has lots of green algae in it. I use sand at the bottom and there is always bright green algae all over the sand, glass and decorations. Do you know what the cause of this is? I change about 15% of his water once a week. I have one small amazon sword in his tank, but the algae grows rapidly... How can I stop this? Also, I have a 10 gallon and it is full of brown algae and I cant get rid of it! Please help!