If you get rid of the guppies then you can clean out the algae completely before a betta. With the guppies in the tank you could just black it out for a few days to get rid of the algae. You really don't need a light for the tank at all, especially not for 14 hours a day. Light just encourages algae growth. Just turn the light on when you want to see the fish better, don't leave it on for any amounts of time.