Angelfish reward a keeper who quarantines. They are long-lived, slow to forgive poor water, and prone to sulking off their food when a tankmate imports trouble, so a disease carried in by one hurried addition can ripple through fish you have raised for years. A plain quarantine tank, a strict rule about shared equipment, and a patient observation period stand between a shop's crowded systems and your tall, deliberate angelfish.
A plain tank, deliberately separate
Set the quarantine tank somewhere away from the display — a different surface, ideally a different room corner — with a bare bottom you can inspect, a heater matched to the angelfish tank's temperature, and a filter sponge that has been kept running in an established tank so it carries working bacteria. Add a short pipe or flowerpot for hiding, because a fish that can hide eats sooner and shows its true behaviour earlier. Bare and simple is the point: every fish, dropping, and leftover pellet stays visible.
Acclimating into quarantine, not the display
Float and blend the newcomers into the quarantine water exactly as carefully as you would into the main tank, since they face the same temperature and chemistry changes plus the strangeness of a sparse box. Discard the bag water, keep the lights low, and let the fish find the hiding tube. Feed lightly from the next day with the same foods they will meet in the angelfish tank, so appetite — your single best daily health signal — is measured against familiar meals.
Hygiene rules the whole house must keep
Quarantine fails through small kindnesses: a net borrowed for one quick job, a siphon shared without drying, wet hands moving from one tank to the other. Give the quarantine tank its own net and bucket, work in the display tank first and quarantine last each day, and wash and dry hands and tools between the two. Drying matters, because most waterborne problems survive poorly on dry equipment. A note on the quarantine lid — who last used which net — sounds fussy until the day it explains, or prevents, an outbreak.
Watching for what angelfish must never meet
Each day, look for eager feeding, clear eyes, smooth fins, and droppings that look normal for the species. White specks, a grey film, rubbing against the pipe, clamped fins, or a fish that stops eating are reasons to keep the quarantine closed and seek advice from an aquatic veterinarian rather than to move the fish and hope the big tank dilutes the problem. Photograph anything odd under the same light each day; change over time is easier to judge from pictures than from memory.
The transfer, once the coast is clear
When the newcomers have fed confidently and shown no symptoms across a full observation period, acclimate them from quarantine water toward display water gradually — the two tanks may have drifted apart in temperature or chemistry — and introduce them with the display lights low. Rearranging a plant or branch beforehand can soften an established angelfish's sense of territory. Writing the quarantine log, observations, and transfer plan in KinsLe Family Sharing keeps every member of the household from accidentally shortcutting a separation that protects the angelfish.