Betta Tank Mates 20 Gallon

Back in August, I purchased my first ever fish. It’s a Male Koi Betta.

I put him in a 3 gallon tank (not knowing any better) but he has had vibrant colors, been happy, making bubble nests, and eating.

Just today I purchased a 20 Gallon tank hoping to add some tank mates

The only tank mate right now is a mystery snail that I purchased along with the Betta. The Betta totally ignores the snail and has no problem with it.

Any ideas on some “starter” tank mates for my betta?

Any help is appreciated!

Thanks in advance!