If your tank is getting too dirty, the LAST thing you want to do is add a snail... they poop like champions and are only good if you want so me good fertilising machines for your plants. Also, 'brown algae' is simply a slang term for microscopic creatures called diatoms. They are COMPLETELY different for algae, and therefore decreasing the light will only make them worse. I can help you more if you tell us
a) How big your tank is
b) Does your betta have any 'room mates'?
c) How often are you cleaning your tank?