Juveniles keep their secret
Angelfish frustrate more new breeders than almost any other community fish, because youngsters of both sexes grow up wearing the same stripes, the same fins and the same shape. There is no colour difference to learn and no reliable size rule: the biggest fish in a young group is simply the fastest grower, not necessarily a male. This is worth accepting early, because money changes hands on guesses — a seller offering guaranteed-sexed juvenile angelfish is selling confidence, not information. What you can do with young fish is keep them well, watch them grow, and let maturity answer the question for you.
The clues that mislead
Two traditional clues circulate endlessly and deserve their poor reputation. The forehead hump — a slight bulge some males develop — appears on plenty of females and is absent in many males, and it varies by strain and condition. Fin length and body size are worse: fins reflect genetics and water quality, and females of a well-fed group may outgrow males. Behaviour misleads too, since young angelfish of both sexes spar, chase and claim corners as they sort a pecking order that has nothing to do with pairing. Treat all of these as entertainment. None of them should decide which fish you buy, sell or pair.
The breeding tube is the real tell
The dependable sign appears when the fish approach breeding condition, commonly in the second half of their first year, though size and condition matter more than birthdays. Just before spawning, the breeding tube — the papilla — drops between the ventral fins and the vent. The female's tube is the wider one: relatively short, blunt and rounded, built to lay eggs onto a surface. The male's is narrower, more pointed and slightly slanted. You will usually see it because a pair has formed and started behaving like one: cleaning a slate, leaf or cone together, chasing tankmates away from it, and staying close. Two fish doing that, with matching blunt and pointed tubes, are as close to certainty as angelfish keeping gets.
If the tubes never show
Occasionally a fish reaches a good size, conditions are right, and no tube ever appears and no pair forms. That usually means the group is short of one sex, the fish are not yet in breeding condition, or a supposed pair is in fact two of the same sex tolerating each other. Rich feeding, clean water and time resolve the condition cases. The others resolve only by adding fish and letting a larger group sort itself out — another reason to buy youngsters in small groups rather than in hopeful pairs.
Work with pairs, not predictions
The practical method is to raise a small group of youngsters together and let a pair choose each other, rather than forcing two guessed fish together. Once tubes show and a pair bonds, record what you have actually confirmed — which fish is which sex, when the tube appeared, when the first spawn followed — because memory blurs fast across a group of near-identical silver fish. That is exactly the kind of slow-building record the KinsLe Pet Passport is for: each angelfish's confirmed sex, pairing and spawning history kept in its own packet, ready when you plan the next generation or hand a proven pair to another breeder.