In my case when I had horrible algae issues (I got them resolved!) for me, it was having too much light, too close. Once I raised the fixture by hanging it from the ceiling from the hardware the company provided, my issues were sort of fixed. I had to do tests to see why I was getting algae. It took patience and trial and error. Now after all that my troubles are gone since I found out how to take car of it and the balance you need.
What kind of lights do you have over the tank? Is it just the stock lights that come with the tank? How long do you keep the lights on for?
The best algae eater is amano shrimp, but depending on where you live and where you buy, they can be expensive or cheap. However it is a hit or miss with shrimp and bettas, also usually bettas will eventually tend to hunt shrimp out, these shrimp do not breed in freshwater. The other option that is also great are nerite snails. With snails usually population will get out of hand, but these snails cannot breed in freshwater, they can lay eggs, but they won't hatch.vWith bettas I would recommend the nerite snails since they have almost nothing that a betta wil be able to grab and kill them (antennae).
15 Gallon NPT
1g no tech bowl in the making