Fins are ragged, split, shortened or discoloured at the edges.
Short answer
Ragged fins are a sign, not a diagnosis. Physical damage, nipping and bacterial fin erosion all look similar at first, and the treatment is different for each. Water quality is the common factor behind almost all of them.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: Damage is on trailing fins only, edges are cleanly torn rather than eroding, and there is a known nipper present — tiger barbs, serpae tetras, black skirt tetras or an under-sized group of any of them.
How to tell: Damage progresses from the fin edge inwards over days, often with a white or reddened margin, and affects several fish. Water tests poorly or maintenance has slipped.
How to tell: A single clean tear on one fish, not progressing. Sharp hardscape, a strong intake or a netting incident.
How to tell: A long-finned betta with symmetrical chunks missing, no other fish present, damage appearing overnight.
Checked before anything else, because a keeper who learns their swollen fish is carrying eggs and their spotted fish is a breeding male has been spared two unnecessary medications.
How to tell it apart: New fin tissue is thin, clear and slightly irregular, and it lengthens the fin week on week. Photograph the fish and compare after seven days: direction of travel answers the question.
Each entry states the observation that separates it from the others, and links to the full record. Work through them before deciding what you are treating.
How to tell it apart: The full account of what eroding fins can mean, and why 'fin rot' is an appearance rather than a disease. Start here before considering any medication.
How to tell it apart: Progression over hours to days rather than weeks, with a pale greyish margin and often a body lesion. Rapid progression is the signal that this is not ordinary erosion — and the response includes cooling rather than warming.
How to tell it apart: A raised cotton-wool growth on the damaged edge, indicating secondary colonisation of tissue something else injured.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.