Question...

[jackie]

You need to change the tank water at least once a week, and you should leave the tank light on about 12-14 hours a day, more then that, and you can get algae problems.

Do not get a chinese algae eater, one betta is the only suitable fish for that tank.

With frequent water changes and a glass scraper the algae should disappear!

[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.