My betta is sick. What do I do first?

Test water parameters. Ninety percent of 'sudden illness' is a spiked ammonia or a dropped temperature. Only after ruling out environmental causes should you consider a specific disease.

Should I use aquarium salt?

For mild external infections and stress reduction, yes. One tablespoon per 5 gallons, dissolved first, for a treatment course of 7 to 10 days. Not for fish with advanced disease. Not as prevention.

Is a hospital tank necessary?

For treating one fish, yes. A 2 to 5 gallon bare-bottom tank with a sponge filter seeded from the main tank, a heater, and no substrate. Keeps medication costs down and protects plants and beneficial bacteria in the main tank.

What's the most commonly misdiagnosed illness?

Fin rot and columnaris. Both produce deteriorating fin edges. Columnaris is more aggressive, kills faster, and needs different medication. Fin edges turning white and fuzzy with visible receding tissue: columnaris. Fin edges turning black or red with slower recession: fin rot.

When the fish can no longer swim upright, no longer eats, and shows no improvement after 72 hours of corrected water and appropriate medication. See /care/humane-euthanasia/ for the protocol.

Betta Disease: Diagnosis and Treatment, Evidence-Based

Most betta disease traces back to a water-quality failure, a temperature drop, or an immune-suppressed fish picking up an opportunistic pathogen. Before you medicate, test parameters and check temperature. Ninety percent of “sudden illness” resolves at that step. For the other ten percent, this page lists the eleven conditions that actually hit pet bettas, organized by how commonly you’ll see them, with treatment protocols cited to the Merck Veterinary Manual, Bacterial Diseases of Fish and peer-reviewed sources, including PMC9334006 (Care and Use of Siamese Fighting Fish, 2022).

The diagnostic sequence, every time

Test ammonia, nitrite, nitrate, temperature. Record numbers.
Correct any issue: water change if ammonia above zero, temperature to 26 °C if cold, remove uneaten food.
Observe 24 hours. Many fish recover from this alone.
If the fish is still symptomatic, match the pattern below.
Treat one condition at a time. Never combine medications without specific pairing instructions.

The differential table

Condition	First signs	Progresses to	Days from first sign to death without treatment
Fin rot	Fin edges ragged, dull	Tissue recedes toward body	30 to 60 days
Columnaris	White fuzzy patch on head/mouth	Rapid tissue loss	3 to 7 days
Ich (white spot)	White pinhead spots on body + fins	Spots multiply, flashing	14 to 21 days
Velvet	Gold/rust dusting on body	Scale loss, lethargy	7 to 14 days
Fungus	White cotton tufts	Secondary infection	Variable
Dropsy	Bloat, pineconing scales	Organ failure	7 to 14 days
Swim bladder	Floating sideways or sinking	Depends on cause	Often resolves
Popeye	One or both eyes bulging	Eye loss, blindness	14 to 30 days
Mycobacteriosis	Wasting, ulcers, spinal curve	Systemic, chronic	Months
External parasite (gill flukes)	Flashing, rapid gilling	Gill damage	7 to 21 days
Ammonia poisoning	Red streaks, clamped fins, surface gasping	Gill damage, secondary infection	Variable

Full details on each condition at the linked articles below.

A marble tricolor Betta splendens showing the unstable color pattern produced by the Kit Ligand A transposable element, a pattern that can be mistaken for stress-related color change. — A marble tricolor male. Marble pattern shifts throughout a fish's life due to transposable element activity in the Kit Ligand A gene, a documented genetic mechanism, not a disease. Distinguishing true color change from disease-related fading requires knowing each fish's individual baseline. A fish that was always marble and shifts pattern is fine; a fish that was solid red and turns pale has a problem worth investigating. *Photo: Thexposeidon via Wikimedia Commons, CC BY-SA 3.0.*

A red traditional plakat male betta showing the short-finned body plan closer to the wild form. — A traditional plakat male. The short-finned fighting lineage is measurably hardier than elaborated show strains: shorter fins heal faster, less surface area for bacterial colonization, better immune profile in recent research. *Photo: Daniella Vereeken via Wikimedia Commons, CC BY 2.0.*

Conditions covered

Bacterial:

fin rot, the most common condition in the hobby. Aeromonas or Pseudomonas colonizing stressed fins.
Columnaris, Flavobacterium columnare, aggressive, fast-moving.
Dropsy, terminal in most cases. Aeromonas internal infection or organ failure.

Parasitic:

ich, Ichthyophthirius multifiliis. Heat treatment protocol.
velvet, Piscinoodinium/Oodinium. Darkness + copper treatment.

Atypical bacterial:

mycobacteriosis, fish TB. Untreatable, chronic, zoonotic. The 2023 peer-reviewed literature is alarming.

Physiological / secondary:

swim bladder disease, often dietary or constipation-driven.

Protocol:

the euthanasia protocol, cross-link to the care-side protocol.

The medication shortlist

Keep these on hand if you plan to keep bettas past the first year:

Seachem NeoPlex (neomycin). Moderate bacterial infections, columnaris.
Seachem KanaPlex (kanamycin). Gram-negative bacteria, advanced fin rot, severe infections.
Hikari Ich-X. Ich and velvet, the most reliable malachite green formulation available over the counter.
API General Cure. Internal parasites, hole in the head, Hexamita.
Aquarium salt (not table salt, not marine salt). Mild external infection support.
Indian almond leaves. Mild antimicrobial, tannin-driven stress reduction.

Seachem KanaPlex kanamycin fish treatment

Seachem KanaPlex
Kanamycin, bacterial Buy on Amazon (affiliate)

Hikari Ich-X
Ich and velvet Buy on Amazon (affiliate)

API General Cure
Parasites, HITH Buy on Amazon (affiliate)

Do not keep these:

Melafix, Pimafix, Bettafix. Tea tree oil-based. Bettas are labyrinth breathers, and tea tree oil irritates the labyrinth organ. Use for other fish or not at all.
Broad-spectrum “prophylactic” treatments dosed into a healthy tank. Medication without a diagnosis is how drug-resistant bacteria arise. Don’t.

The hospital tank

A hospital tank is a 2 to 5 gallon bare-bottom tank with a sponge filter, preset heater, and no substrate. You keep it cycled by seeding with a sponge from the main tank filter. When a fish needs medication, you move the fish into the hospital tank and treat there.

Why:

Medication kills beneficial bacteria. Treating the main tank crashes the cycle.
Meds kill invertebrates (copper kills shrimp, kanamycin kills snails).
Bare bottom makes it easy to see and siphon waste during treatment.
Small water volume cuts medication costs dramatically.

How:

Keep a second sponge filter in the main tank at all times, running.
When needed, move the seeded sponge and the fish to the hospital tank.
After treatment (usually 7 to 14 days), return the fish.

When medication isn’t the answer

Water changes. Fresh, warm, temperature-matched water fixes more problems than any single medication. If you’ve forgotten everything on this page, daily 50% water changes for a week with Prime-treated water is a valid treatment for 60% of presentations. Boring, unglamorous, works.

A healthy halfmoon Betta splendens with full fin extension and clear body coloration, the reference baseline against which every disease presentation is compared. — A healthy halfmoon male: clean fin edges, fully extended spread, saturated color. Every disease article on this site uses this as the reference baseline. Fin edge quality, color depth, and body posture are the three fastest early-warning reads, any departure from this individual fish's normal requires investigation before it becomes a diagnosis. *Photo: Ar-betta via Wikimedia Commons, CC BY-SA 4.0.*

Every article linked above covers the specific pathogen, specific medication, specific dosage. But the underlying discipline is: test first, fix environment first, then diagnose, then medicate, then hospital-tank, then evaluate. Keep that order and you won’t lose many fish.

Frequently asked

My betta is sick. What do I do first?: Test water parameters. Ninety percent of 'sudden illness' is a spiked ammonia or a dropped temperature. Only after ruling out environmental causes should you consider a specific disease.
Should I use aquarium salt?: For mild external infections and stress reduction, yes. One tablespoon per 5 gallons, dissolved first, for a treatment course of 7 to 10 days. Not for fish with advanced disease. Not as prevention.
Is a hospital tank necessary?: For treating one fish, yes. A 2 to 5 gallon bare-bottom tank with a sponge filter seeded from the main tank, a heater, and no substrate. Keeps medication costs down and protects plants and beneficial bacteria in the main tank.
What's the most commonly misdiagnosed illness?: Fin rot and columnaris. Both produce deteriorating fin edges. Columnaris is more aggressive, kills faster, and needs different medication. Fin edges turning white and fuzzy with visible receding tissue: columnaris. Fin edges turning black or red with slower recession: fin rot.
When do I euthanize?: When the fish can no longer swim upright, no longer eats, and shows no improvement after 72 hours of corrected water and appropriate medication. See /care/humane-euthanasia/ for the protocol.