Question...

[Jessika]

If you have an algae problem its probably because the tank is receiving too much sunlight. Try moving it away from a window or into another room?

I think a betta would thrive in a 2.5 gallon tank. You realized your mistake with the guppies and are trying to fix the problem, so that shows you have learned something from the ordeal!

[IRescue452]

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.