I wouldn't go with an ADF as there is a risk of the Betta and them not getting along and then you have to worry about injuries or one killing the other.
Mystery Snails are neat little guys. Many different colors. And they don't eat up healthy plants. They only eat them if you don't feed them! They in fact keep plants healthy because they clean up dead plant matter. Also, this snail comes in male and female, so no babies if you only have 1.
Shrimp are great tankmates and good little cleaners with a VERY low bioload (about 5 per gallon!). Ghosts are Cheaper than Cherrys, but have a higher mortality rate because of being bred to be feeders. Just make sure to give him a place to hid just in case he decides they are a snack
If I were you, I would do maybe 1 small snail and 5-10 shrimp or just shrimp.
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