Betta fin rot: how to tell, treat it and get the fins to grow back

Betta fin rot is almost always a water problem first and a bacteria problem second. Move your betta into clean, warm, filtered water (a heated tank of at least 19 litres / 5 gallons), and most early fin rot stops spreading within a week without any medicine.

Bettas get fin rot more than most fish because their long, heavy fins tear easily and give bacteria a lot of surface to work on, and because they are so often kept in small, cold, unfiltered bowls. This guide is betta-specific. For fin rot in other fish, see fin rot.

How to identify it

Fin rot

Fin edges look ragged, shredded or "melted", and the fins slowly get shorter.
The edge is often discoloured: black, brown, red or milky white.
The damage creeps inward from the edges over days.
In bad cases, red streaks run through the fins, or the rot reaches the body.
The betta may also clamp its fins, hide and eat less.

Fin biting (tail biting)

Pieces go missing overnight, often in a crescent or bite shape.
Edges are clean, with no dark or white rim.
Usually the tail fin, and often in long-finned males with heavy tails.
Often linked to boredom, stress or a too-warm tank. The big danger is that the bitten edges then get infected.

Snags and tears

Sharp plastic plants and rough ornaments rip betta fins. A tear with a clean edge and no colour change is an injury, not rot. Run a pair of tights over your decor: anything that snags the fabric will snag fins.

Normal fin curl and old age

Some bettas, especially halfmoons and crowntails, have fins that curl or fold at the edges because of genetics or warm water during growth. Curled fins with healthy, clear edges are not rot. Older bettas also have thinner, droopier fins.

Is it columnaris?

If you see white, cottony patches on the body, mouth or back, or the damage spreads within hours rather than days, it may be columnaris, which is more dangerous and needs faster treatment.

When it's a problem

Early: slightly ragged or dark edges, the betta still active and eating. Clean water usually fixes this.

Serious: the fins have lost a quarter or more of their length, the edges look bloody or fuzzy, or the rot keeps advancing after a week of clean water.

Emergency: the rot has reached the body, there are red patches or ulcers on the skin, the scales stick out (see dropsy), or the betta lies on the bottom and won't eat. Act within hours.

What to do right now

Test the water for ammonia, nitrite and nitrate. In small bowls, ammonia is usually the culprit.
Do a large water change (50% or more) with dechlorinated water matched to the tank temperature.
Check the temperature. Bettas need about 25–27°C (77–80°F). Cold water slows healing and weakens their immune system.
Remove sharp decor and anything that snags.
Photograph the fins against a plain background so you can compare in a few days. Rot and healing are hard to judge by memory.
Feed small meals, and remove uneaten food after a few minutes.

What to do next

Fix the home: this is the real cure

The root cause of most betta fin rot is a small, unheated, unfiltered bowl. In a one-litre jar, ammonia builds up within a day or two, and the temperature follows the room. Move your betta to a tank of at least 19 litres (5 gallons) with:

an adjustable heater set to about 26°C (78°F) and a thermometer,
a gentle filter (a sponge filter is ideal; bettas struggle in strong current),
silk or live plants, a cave and a resting leaf near the surface,
a lid, because bettas jump.

A 2024 study found bettas in a 19-litre furnished tank were more active and showed fewer abnormal behaviours than bettas in jars and small tanks. If the new tank is not cycled, test daily and change water whenever ammonia or nitrite shows up; see cycling.

Treatment escalation

Step 1 (days 1–7): clean water only. Change 25–30% of the water every two or three days in a filtered tank (daily in an uncycled one). Keep the temperature steady. Many cases stop here.

Step 2: optional aquarium salt. Some keepers add aquarium salt at a low dose (about 1 g per litre) for up to 10 days to help the slime coat. Experts disagree on how much it helps. Don't use it with live plants you care about, and replace the salt only for the salt removed in water changes.

Step 3: medication, if the rot is still spreading after a week or is moving fast. Use a hospital tank if your betta shares a tank. Remove carbon from the filter, because it absorbs medicine. Look for a product for bacterial fin and tail rot. Over-the-counter options vary by country. Many antibiotics sold for fish in the US need a vet prescription in Canada, the UK, the EU and Australia. Follow the label exactly, finish the full course even if the fins look better, and don't mix medicines.

Step 4: fuzzy edges. If the fin edges grow white fluff, a secondary fungal infection may have set in. See fish fungus.

If your betta lives with snails or shrimp, move the betta to treat. Many fin rot medicines harm invertebrates and plants.

Regrowth: what healing looks like

Healing fins grow back with a clear, thin, see-through edge. That clear tissue is new fin, not rot. It can take a few weeks to show and months to fully regrow. Regrown fins may be a slightly different colour, wavier or less perfect than before. If the edge turns dark again, rot has returned: go back to step 1 and check the water.

Products that help

Adjustable aquarium heater (25–50 W for small tanks): keeps your betta at the right temperature; the single best upgrade from a bowl.
Sponge filter and small air pump: gentle filtration that bettas can swim against.
Liquid test kit: tells you if ammonia or nitrite is the cause.
Water conditioner: removes chlorine and chloramine with every change.
Silk or live plants: replace sharp plastic decor.
Aquarium salt: optional supportive treatment at low doses.
Fin and tail rot antibacterial medication: for rot that keeps spreading after a week of clean water. Check which active ingredients are legal without a prescription where you live.
Small hospital tank or tub with a heater: lets you treat without harming tankmates or plants.

How to prevent it

House each betta in a heated, filtered tank of 19 litres (5 gallons) or more.
Change 20–30% of the water weekly and test regularly.
Keep the temperature steady at about 26°C (78°F).
Avoid fin-nipping tankmates such as tiger barbs and some tetras.
Choose smooth decor and plants with soft leaves.
Don't overfeed; leftover food rots and pollutes small tanks fast.

Why it happened

The bacteria that cause fin rot live in almost every tank. They take hold when fins are damaged and the fish is weakened by dirty water, cold or stress. Fix those and the betta's own defences usually do the rest.

When to get help

Get help if the rot reaches the body, if you see ulcers, swelling or raised scales, if it spreads fast despite clean water, or if two weeks of treatment haven't stopped it. An aquatic vet can confirm the cause and prescribe the right antibiotic where one is needed.

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