Bettas don't have to be in tank with tank mates, but i understand wanting to add more fish to a large aquarium. The cherry shrimp are good, I've never had any, but many people on here have. Ghost shrimp are good as well, though depending on the betta they may try to bully them. Neon tetras are good, though they can fin nip so watch out for the betta and they are schooling fish so 4+ is a must, but 6+ is better. Snails are good too, I have a mystery snail and love it, other types of snails I dont know much about, except for research them well before getting one because some become more of a pest than a pet. Cory catfish are also something that many betta keepers will put in with their fish, though as far as personal experience with these, I dont have any.
Also, not really a fish, but moss balls are fun, they will move and follow light occasionally and your shrimp will love to clean them.
EDIT: I feel that two other things need mentioning, one is that when adding fish to a tank only add them in three's or it may cause major and unhealthy fluctuations in the bioload as well as when adding fish, know that bettas can be agressive and may not always tolerate other fish well so always have a back up plan in case things go south (this is kinda two things, but hey...). Second NEVER put another betta in the tank, whether it be male or female. Neither males or female and male will live together, they will fight and you will end up with dead/injured fish.
peace. love. & bettas.
Last edited by Loralyn94; 08-12-2011 at 08:16 PM.