Betta fin rot, and what actually treats it
Three steps in a fixed order, and the first one is free. The reason the order matters is that two of the three stop working if you skip it.
We earn a commission if you buy through the links on this page, at no extra cost to you. It never changes a ranking, and there are products recommended here with no affiliate link behind them at all. Full disclosure.
The answer, in order
Fix the water first, and in most cases that is the whole treatment. Fin rot in a small tank is an opportunistic bacterial problem that takes hold when the water is wrong, and the water being wrong is the part you can act on today. Test for ammonia and nitrite, change enough water to bring them to zero, and keep them there daily until the fin stops receding.
Add aquarium salt second, at the published rate - about two rounded teaspoons for a full 4.3-gallon volume. API publishes half a rounded teaspoon per US gallon, and publishes in the same breath that salt “is not a holistic solution to heal and treat fish diseases”. That is the correct way to read it: a cheap supporting measure on top of clean water, not a cure.
Medicate third, and only if the fin is still retreating after about a week of clean water and salt. API Fin & Body Cure is the product that names fin and tail rot on its own label and publishes a complete four-dose course. It is the right third step and the wrong first one.
If the water tests dirty, stop here
Ammonia or nitrite above zero is the diagnosis. No medication on the shelf treats it, every antibacterial course asks you to remove the carbon from your filter, and dosing a tank that is already failing chemically makes the week worse rather than better. Change the water, keep changing it, and come back to this page when the test reads zero.
Rot, or a torn fin? The distinction that decides everything
These two look identical in a photograph and they always will. What separates them is time:
- A tear has a clean edge, appears between one day and the next, and gets smaller as the fin regrows. A betta that caught a fin on rigid decor or a filter intake has a mechanical injury, and healthy water is the entire treatment.
- Rot advances. A little more is gone each day, the margin between healthy tissue and damage is often dark or reddened, and the edge looks ragged or melted rather than cut. Progression over three days is the observation that justifies treating.
So: photograph the fin against the glass today, fix the water, photograph it again in three days. If the gap has not grown, you had a tear and you are already done. This one step prevents most of the medication this hobby pours into 5-gallon tanks.
If it was a tear, the fix is the tank. A sponge over the filter intake costs about a dollar a sponge, and the fake-plant question covers which decor does the tearing. Both are cheaper than a course of anything.
Step one: the water, and how much of it
The useful arithmetic here is the same one that governs every dose on this site. A rated 5-gallon holds roughly 4.3 gallons of actual water once substrate and hardscape have taken their share, so a 50% change moves about 2.15 gallons and needs 0.22 mL of Prime at Seachem’s published rate.
| Tank (rated) | Actual water | Prime, full volume | Prime, 50% change | Stability, day one |
|---|---|---|---|---|
| 3 US gal | 2.6 gal | 0.26 mL | 0.13 mL | 1.3 mL |
| 5 US gal | 4.3 gal | 0.43 mL | 0.22 mL | 2.15 mL |
| 10 US gal | 8.5 gal | 0.85 mL | 0.43 mL | 4.25 mL |
| 20 US gal | 17 gal | 1.7 mL | 0.85 mL | 8.5 mL |
During active fin rot we would do that 50% change daily rather than weekly, and keep doing it until the fin edge stops moving. The usual objection is that large daily changes stress the fish; in a tank this size the alternative is the fish sitting in the water that caused the problem, and the normal maintenance schedule is built for a healthy tank rather than a failing one.
Match the temperature of the new water to the old. A nano tank has so little thermal mass that a careless bucket swings it several degrees, and a temperature swing on a fish already fighting an infection is a second problem stacked on the first.
Step two: salt, and exactly how much in a nano tank
API publishes one rounded tablespoon per 5 US gallons, or half a rounded teaspoon per gallon. On 4.3 gallons of real water that is about two rounded teaspoons for a full-volume dose, or about one rounded teaspoon mixed into the new water of a 50% change.
Two published details decide how you use it. The first: salt does not evaporate and is not removed by the filter, so API’s own direction is to add it only with changed water. Topping up a tank that has evaporated and re-dosing salt is how a mild salt level becomes a strong one without anyone deciding to make it so. The second: API publishes a plain caution about live plants, and in a planted nano there is nowhere for the salt to dilute to.
The figure nobody publishes
How long a tank may be kept salted. API gives a dose and no maximum duration, and the ten-day ceiling repeated across forums appears on no label we could find. We will not print a number the manufacturer declines to. What we will say is that salt is a treatment phase, not a permanent water parameter, and the obvious end point is a fin that has stopped receding - at which point ordinary water changes dilute it out on their own.
Step three: the medication, and why this one
Three products get bought for fin rot and they are not equivalent. The difference is what the manufacturer is willing to print.
| Product | Names fin rot? | Published dose | Course | Active ingredient |
|---|---|---|---|---|
| API Aquarium Salt | No - sold as a supporting measure | 1 rounded tbsp / 5 gal | Re-dosed with water changes only | Salt (evaporated seawater) |
| API Fin & Body Cure | Yes - “fin and tail rot”, by name | 1 packet / 10 gal | 4 doses, 2 water changes, carbon out | Not published |
| API Melafix | Yes - listed among several indications | 5 mL / 10 gal | Daily for 7 days, then 25% change | Not published |
Fin & Body Cure is the one we would use, because it is published as a medication with a defined end: four doses, a 25% water change after the first two, another 24 hours, a final 25% change and fresh activated carbon. API also states that the carbon has to come out of the filter during treatment, because it strips the dose - an instruction most remedy labels leave out and most keepers therefore skip.
The nano-tank problem with it is the packet. One packet treats 10 gallons and the powder cannot be halved accurately by eye. On a 5-gallon the honest options are to treat in a separate 10-gallon-equivalent of water, or to accept the imprecision of splitting a packet, and we would rather say that than pretend a sachet divides.
Melafix is ranked last here on evidence, not on price. API lists fin and tail rot among its indications and publishes a seven-day course, and also names no active ingredient anywhere on the page and cites no efficacy data of any kind. The only chemical named on it is beta-myrcene, in a Proposition 65 warning. That does not make it useless - it makes it unevaluable, and seven days spent on an unevaluable remedy is seven days a receding fin does not have.
What causes it in the first place, which is the part that stays fixed
- 01
Water that is not changed often enough for the volume
A 5-gallon with one betta needs a weekly routine, not a monthly one. This is the cause in most cases, and it is also the one that recurs if only the fish gets treated. - 02
A filter that cannot keep up, or was never cycled
A tank put into service before the biological filter established runs ammonia for weeks. The fishless cycle page exists for this reason, and a fin-rot case in a tank under two months old should be read as a cycling failure until the test kit says otherwise. - 03
Decor and intakes that tear fins, which then get infected
Rigid plastic plants and bare impeller intakes are the two usual culprits. A tear in clean water heals; a tear in dirty water becomes the thing you are now reading about. - 04
Temperature below the range the fish can heal at
A betta held in the high sixties has a slowed immune response and a slowed metabolism, and a fin will not regrow at a useful rate. The temperature page sets out the range; a thermometer is the only way to know you are in it. - 05
Overfeeding, which is a water problem wearing a different hat
Uneaten food in a nano tank is ammonia on a delay. The feeding-frequency page covers the portion; the point here is that the fin damage and the full stomach often have the same root.
What we cannot tell you
Whether your fish has fin rot. The distinction from mechanical damage needs three days of observation, not a photograph, and we would be inventing confidence we do not have by saying otherwise.
We also cannot tell you what is in two of the three products on this page. API publishes no active ingredient for either Fin & Body Cure or Melafix, which means nobody outside the company can evaluate either one on its chemistry. We record that rather than filling the gap, the same way we record a filter that will not publish a flow rate.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and the arithmetic they imply - not on testing we have not done.
API Aquarium Salt, 36 oz
The first thing to reach for and the cheapest, and API is unusually straight about its limits. The published dose is one rounded tablespoon per 5 US gallons, or half a rounded teaspoon per gallon, and because salt neither evaporates nor filters out, API's own direction is to add it only with a water change rather than topping it up. API also publishes two sentences the forums drop: that it does not change hardness or pH, and that it is "not a holistic solution to heal and treat fish diseases". Treat it as the supporting measure it is sold as, on top of clean water, and it is the best six dollars in a betta cabinet.
Best for: The first response to ragged fins, and the one thing worth owning before anything is wrong
Skip it if: Your tank is heavily planted and you are not prepared to lose plants. API publishes a plain caution to take extra care with live plants, and in a 5-gallon there is nowhere for the salt to dilute to.
API Fin & Body Cure, 10-packet box
The one product in the group that is sold as an actual medication and published as one. API lists fin and tail rot by name, prints a four-dose course rather than a vague "repeat as needed", and tells you to pull the carbon out of the filter because it will strip the dose. One packet treats 10 gallons, which is the awkward part on a 5-gallon tank: the packets are not divisible with any accuracy, so a nano keeper either doses half a packet by eye or treats a hospital container at the stated rate. API also carries a Proposition 65 warning for crystalline silica on this product, which is worth reading before you open a powder packet over an open tank.
Best for: Fin rot that is still advancing after a week of clean water and salt
Skip it if: The fins are receding but the water tests clean and the edges are not reddened or fuzzy. API publishes an indication list, not a reason to medicate on suspicion, and a four-dose antibacterial course in a 5-gallon is not a precaution.
API Melafix, 4 oz
The bottle almost everyone buys first, and the one we rank last of the three on what is published rather than on what it is rumored to do. API lists fin and tail rot among its indications and prints a clear course - 5 mL per 10 gallons, daily for seven days, then a 25% change - and states it will not harm the biological filter or shift pH. What the page does not do is name an active ingredient, and it publishes no efficacy evidence of any kind; the only chemical named anywhere on it is beta-myrcene, in the Proposition 65 warning. That is not a reason to call it useless. It is a reason not to let it replace the water changes that do the actual work, and not to spend a week on it while fins keep receding.
Best for: Minor fin damage on a fish in clean water, where the honest alternative is doing nothing
Skip it if: You want to know what you are putting in the tank. No active ingredient is published, so there is nothing to judge the dose against and nothing to look up.
Frequently asked
Will fin rot heal on its own?
In clean, warm, stable water, mild cases frequently stop advancing and the fin regrows over several weeks. That is the reason the order on this page puts water first: the step with no cost and no risk resolves a large share of cases before a medication is needed.
How much aquarium salt for a 5-gallon betta tank?
API publishes half a rounded teaspoon per US gallon. A rated 5-gallon holds about 4.3 gallons of actual water, so a full-volume dose is about two rounded teaspoons, or about one teaspoon mixed into the new water of a 50% change. Add it only with changed water - API states that salt neither evaporates nor filters out.
Can I use aquarium salt in a planted tank?
API publishes a plain caution to take extra care with live plants, and does not quantify it. In a planted nano there is nowhere for the salt to dilute to, so the realistic choices are treating the fish in a separate container, or accepting that some plants may suffer. Anubias and java fern are generally tougher about it than stem plants; we cannot give you a concentration that is safe for a given species because nobody publishes one.
Is Melafix or Bettafix enough for fin rot?
Possibly, and it cannot be established from anything published. API lists fin and tail rot among Melafix’s indications, prints a seven-day course, and publishes neither an active ingredient nor any efficacy evidence. Bettafix is the same idea in a smaller, more dilute bottle. If the fin is still receding after a week, move to the product that names the condition and publishes a defined course.
Do I need to remove the carbon from my filter?
For Fin & Body Cure, yes - API states that activated carbon removes the medication, and instructs you to take the carbon or cartridge out and keep aeration running for the course. Put fresh carbon back at the final water change. Note that removing the cartridge from some nano filters takes most of the biological media with it, so move that media into the tank rather than discarding it.
How long does fin rot take to heal?
API’s only published timing claim, on Melafix, is that fin and tissue regrowth usually occurs within a week. In practice the useful milestone is earlier and more specific: the edge should stop advancing within a few days of the water being right. Full regrowth of a long-finned betta’s caudal fin takes considerably longer, and the new tissue often comes back clear before it colors up.
Where the figures on this page came from
- Seachem Prime - published dosing directions
- Seachem Stability - published dosing directions
- Aquarium Co-Op, Betta Fish Care Guide - published temperature and tank-size guidance
- API AQUARIUM SALT - published directions, sizes and the manufacturer's own statement that it is not a complete disease treatment
- API FIN & BODY CURE - published indications, four-dose course, packet dosing and Proposition 65 warning
- API MELAFIX - published indications, 7-day course and dosing (the page names no active ingredient and publishes no efficacy evidence)
- API Freshwater Master Test Kit - published product listing