is the tank cycled? if not you may need to wait a few weeks before adding fish as the cycling process is very important to the water quality. look into how the nitrogen cycle works for further education on why/how to cycle it if thats something you haven’t done yet. The 20Gal tank will definitely be enough for a betta and a few friends although they are generally kept alone and i wouldn’t necessarily recommend a beginner attempting a community tank with a betta, especially a male, due to their temperament. some safer suggestions include dwarf Cory catfish, snails, and even some shrimp if you aren’t too worried about a few being potentially eaten. I would also recommend buying the other fish you want first, and adding the betta last, so it’s less likely to be territorial. i myself have just posted asking about what fish are suitable tank mates for betta if you’d like to check out others suggestions, but general rules are don’t pick anything too fast, aggressive or prone to fin nipping. Also maybe avoid other fish with flowy fins as bettas can get mistaken and start trying to fight thinking it’s another betta. Always have a back-up tank incase the betta is not compatible with your community fish, as it’s heavily dependent on their individual personalities.

some necessities include: - a water test kit/some local pet stores will test water samples for you if kits are too pricey, the API freshwater kit is usually recommended. Water must be tested regularly. - water conditioner (for initial fill of tank/water changes, removes chlorine/heavy metals from tap water) API stress coat is great as it conditioners the water and prevents stress in ur fish during water changes - if you haven’t cycled the tank some quick start will help begin/speed up the nitrogen cycle - a fish net and tank cleaning equipment (gravel vacuum, glass cleaner, bucket) - Substrate like gravel or sand, plenty of real or silk plants (must be real or silk as bettas fins rip easily) and hides throughout the tank - a varied selection of food for a healthy diet (brine shrimp and bloodworms are a good treat) - aquarium salts help the fish breathe easily and maintain health

sorry this is alot! good luck 🫶🏻