Mistake one: buying the baby, not the adult
Angelfish are sold as silver-dollar-sized juveniles, all eyes and trailing fins, and beginner mistakes begin with planning for that fish. The adult is a different animal: a deep, tall cichlid whose body and finnage can span the height of a hand, living for many years and claiming a territory once mature. Owners who stock, decorate and size a tank for the juveniles find the whole plan straining within the year, and the upgrades, reshuffles and rehomings that follow are the true cost of the species. Shopping for the adult, in tank, companions and budget, is the single change that prevents much of the rest of this list.
Mistake two: tankmates chosen by looks
Two pairing errors repeat constantly. The first is housing angels with fin-nippers, tiger barbs being the textbook example, whose attention turns long fins into ribbons and keeps the angel permanently stressed. The second is the opposite misjudgment: keeping grown angelfish with very small fish such as tiny tetras or juvenile livebearers, and discovering that a peaceful-looking adult angel regards them as food. Pairs add a third wrinkle, because a bonded breeding pair can turn the whole tank into defended territory and bully every other resident. Successful angelfish communities are built from calm, mid-sized companions chosen for temperament, not from whatever looked pretty in the next shop tank.
Mistake three: a tank that is wide but not tall
Angelfish are shaped vertically, so they need water depth that ordinary community tanks often lack. A long, shallow aquarium gives them swimming room in the direction they use least and cramps the finnage they carry above and below the body, and crowded vertical space sharpens territorial friction between multiple angels. New owners also commonly overstock, treating angels as one more small fish in a busy community, when each adult needs genuine room. The result is the familiar slow decline: torn fins, hiding fish, water quality slipping under a load the filter was never sized for, all blamed on the species rather than the floor plan.
Mistake four: water and feeding that miss the mark
Angelfish are warm-water fish, happiest in the upper seventies to low eighties Fahrenheit, and tanks kept at the cool end of community practice leave them sluggish and vulnerable to every passing infection. Feeding causes the other half of the trouble: angels beg convincingly, and generous owners deliver rich food until the water fouls, when a measured staple diet with occasional variety suits them far better. They also startle easily in bright, bare tanks, so planting and structure are welfare items rather than decoration. None of these needs is exotic; they are simply specific, and a new owner who learns them early keeps a fish that can thrive for a decade.
The pattern behind the mistakes
Every error above is the same error wearing different clothes: researching the purchase instead of the pet. The adult angelfish, its temperature, its tankmates and its territory, makes a short and learnable list, and an owner who covers it before the first fish comes home skips nearly all of the species' beginner grief. For a pairing or a depth question about an adult angel, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.