The short answer for an adult angelfish
An adult angelfish needs at least a 29-gallon tank, and that tank should be tall rather than long and low. A grown angelfish reaches about six inches in body length, but its flowing dorsal and anal fins make the whole fish far taller than it is long. In a shallow tank those fins fold, drag, and fray. A height of around eighteen inches or more gives an adult angelfish honest swimming room from substrate to surface.
Angelfish are cichlids, and two adults who decide they dislike each other will use every inch you give them. For a pair you intend to keep long term, or a small group, moving up to a 55-gallon footprint buys peace that no divider can match.
Why height beats length for this fish
An adult angelfish hunts and hovers in the middle water column, hanging almost motionless before a short, deliberate strike. Its body plan is a tall disc, built for slipping between vertical plant stems, not for cruising a long open lane the way a danio does. A long, low tank wastes volume near the glass bottom while squeezing the vertical space this fish actually occupies.
Tall tanks also soften angelfish tempers. Vertical layers — open surface water, a mid-level planting band, and shaded lower corners — let a subordinate fish drop out of a dominant fish's sightline without leaving the tank.
Setting up the minimum tank well
In a 29-gallon adult angelfish tank, spend the height deliberately. Anchor tall, soft planting or smooth vertical decor toward the back and sides so the center stays open for hovering. Keep the substrate fine and the lower third mostly clear, because an angelfish picks at the bottom between meals and dislikes sharp gravel edges near its ventral fins.
Filtration should turn the water over gently. A grown angelfish in a tall tank is a poor match for a strong, blasting current: those long fins act like sails, and a fish fighting flow all day eats more and rests less. Baffle a strong filter outlet or aim it along the back glass so the front viewing area stays calm.
Lighting deserves the same moderation. Floating cover or taller growth near the surface breaks up bright, exposed light and draws the fish back into open water.
Stocking honestly around the minimum
A 29-gallon angelfish tank is a minimum, not a community invitation. Every tankmate added shares the same water the angelfish breathes, and small, slow fish may be viewed as food by a large adult, while fin-nipping species will shred the very fins the tall tank was chosen to protect. Choose companions that occupy the bottom calmly, grow too large to be swallowed, and leave long fins alone.
Water stability matters more as stocking rises. A single adult angelfish in 29 gallons forgives a slightly late water change; a crowded one does not. Keep a regular partial-change rhythm, feed measured amounts that disappear within a couple of minutes, and watch the fish's posture — an angelfish that stops hovering in the open and camps in one corner is telling you the room, the company, or the water no longer suits it.
When to size up sooner
Plan an upgrade early if your adult angelfish shares with a growing pair bond, if fin edges keep splitting despite gentle flow, or if chasing becomes a daily pattern rather than an occasional flare. Moving a settled adult is easier before stress shows up as appetite loss or clamped fins, and a larger tall tank often settles behavior that no rearrangement of decor could fix.
Keeping the whole household aligned on the upgrade plan, the current dimensions, and who feeds on which day is simpler when KinsLe Family Sharing holds this angelfish's profile where every helper reads the same details.