One Angel or a True Pair, Nothing Between
Twenty gallons is where angelfish keeping genuinely begins, and the stocking that works is precise: a single angelfish, or two that have already bonded as a breeding pair. What does not work is the in-between option most people reach for, two juveniles bought together in the hope they will get along. Angelfish are cichlids with a strict hierarchy. A bonded pair has settled that hierarchy into partnership, cleaning a surface together and defending it as a team. Two unbonded fish have settled nothing, and in twenty gallons the loser has nowhere far enough to retreat to, so the chasing never really stops.
The Shape of the Tank Decides Half the Question
Twenty gallons comes in two very different shapes. The tall model gives an angelfish the vertical water its deep body and long fins are built for, fins clear of the substrate and the surface. The long, low model spreads the same volume out in a way that suits tetras better and leaves an adult angel feeling the ceiling. If your 20-gallon is the tall shape, a single fish or a bonded pair is a fair plan. If it is the long shape, keep it to one angelfish and choose tankmates that use the length, and even then expect the fish to look large in it by its second year.
Building a Community Around a Single Angel
A lone angelfish makes a superb centrepiece, and the community should be chosen to leave it unbothered. Schooling fish that are too large to be eaten and too calm to nip, such as rummy-nose or lemon tetras, share the middle water peacefully. Corydoras work the bottom where the angel rarely bothers to look. Avoid anything with a reputation for fin-nipping, because those trailing fins are an invitation, and avoid very small fish like young neon tetras, which an adult angel may regard as food. Stock the school first and add the angelfish last and smallest, so it grows up treating the others as furniture rather than arrivals to be challenged.
If Breeding Is the Real Goal
A bonded pair in a 20-gallon will spawn, but raising them is where the size starts to pinch: the pair defends its corner against every tankmate, and you will need somewhere for growing fry. The usual route is to start with a group of five or six juveniles in a 40-gallon or larger, let two pair off naturally, and then move or keep that pair as your breeders. Trying to shortcut that by forcing two strangers into a 20-gallon produces fighting, not fry. Treat the 20-gallon as an excellent home for one angel, or a proven pair you bought as a pair, and let breeding ambitions wait for the bigger tank.
The Decision in One Pass
Stand in front of the tank and picture the adult fish: six inches of body, fins trailing well beyond it, wanting a corner to call its own. One fits that picture with room to spare; a proven pair fits it snugly; two hopeful strangers do not fit it at all. If choosing between a lone centrepiece with tankmates and an already-bonded pair leaves you torn, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.