Few things alarm an angelfish owner like finding two fish locked mouth to mouth in the middle of the tank, apparently wrestling. The behaviour is identical at first glance in courtship and in aggression, because it serves the same purpose in both: it is a strength test. Two fish grip lips and push, measuring each other. What the test means — a partner being assessed, or a rival being beaten — is written in everything that happens around it.
The courtship version, frame by frame
In courtship, lip-locking is a negotiation between equals, and it looks surprisingly orderly. The two fish approach face to face, often after parallel swimming and mutual fin displays, lock briefly, push without violent shaking, and release. Crucially, neither leaves. They stay together afterwards, frequently returning to a leaf or slate they have been cleaning, and the locking might repeat a few times across an evening in among nudging and quivering. Both fish keep their colour and their appetite, and both direct their aggression outward, at other fish that wander near their corner.
The fight version, frame by frame
A fight escalates where a courtship settles. The locking is harder, with real shaking and twisting, and it is surrounded by behaviour courtship never shows: high-speed chases around obstacles, ramming, and one fish progressively giving ground. Afterwards the loser does not stay — it retreats to the surface corner or behind plants, pales, clamps its fins and stops competing for food, while the winner patrols and renews the attack whenever the loser shows itself. Torn fins, split tails and a fish breathing hard in hiding are fight evidence, and locks that happen more and more often, rather than resolving into pair behaviour over a day or two, are heading the same way.
Read the trend over forty-eight hours
A single lock proves nothing either way, so judge the trend. Courtship trends toward togetherness: more cleaning, more joint territory defence, spawning behaviour within days. Fighting trends toward separation: one fish exiled, eating less, looking worse each morning. Keep brief notes of what you see morning and evening — who initiated, who retreated, whether they fed together — because two days of notes will tell you more than any single dramatic moment at the glass.
When to step in
Do not intervene in an even courtship test; separating a forming pair repeatedly can prevent the bond entirely. Step in when the evidence turns one-sided: visible fin damage, a fish that cannot come out to feed, one that spends the day hiding or sits at the surface gasping. A tank divider is the least stressful first move, keeping both fish in familiar water while you decide whether the loser needs its own tank permanently. If a fish is injured — open sores, badly shredded fins, a damaged mouth that stops it eating — it needs treatment advice from an aquatic vet, not patience. For a genuinely ambiguous bout, where the hour after the lock points both ways, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.