Dealing/treating suspected Cotton Fin Fungus?


Hey all,

Tank newbie here.

We got a 20 gallon tank from PetSmart, some fish, and were kind of left on our own. Did some reading and research, and have spent the past month successfully cycling our tank. Numbers are all around 0, except for Nitrite, which is 0.5 ppm. Currently have three plants, one glowlight tetra, and two java loaches (they're in there somewhere!).

Last week:
We lost two glowlight tetras to what we think was cotton fin fungus. I did many searches online, and even spoke to a specialty store in the area. They recommend not to dose with an antibacterial product, as it would possibly ruin/reverse the cycle, continue our maintenance, euthanize if necessary, and not to add any more fish for at least two weeks. We've done so and unfortunately had to say goodbye to two fish.

The remaining glowlight tetra appears to have cotton fin fungus too. A very, very light patch on the left-hand side of it's head, and the very tip of its nose is pale too. I've read through this thread on the topic, and am aware of medicines like Pimafix, or treatments like aquarium salt.

I am honestly looking for advice and would rather not have to euthanize the remaining tetra, or risk a spread to the java loaches. I know that aquarium salt can be harmful to loaches, so am very hesitant to use it. I also don't want to guess and use the wrong medicine and hurt the whole tank. If anyone has any recommendations, suggestions, or needs more information from me, please let me know and I will try to answer you as best as I can.

Rose of Sharon


Sorry you are having these issues!!!

If you could set up something that can act like a hospital tank, you could try to medicate the tetra, if you want to try to save it.

I use a 2 gallon clear glass cookie jar. I have a small heater and filter (like a very small sponge filter) that I keep on hand for this purpose. Some people use plastic totes. It's good to have something to use in case you have fish that get ill in the future, and you need to isolate them from the rest of the tank to prevent the spread of disease, or to medicate. It would also be good to use to quarantine new fish until you are sure that they are disease-free.

You could do small water changes daily, so you wouldn't have to worry about the hospital tank being cycled. Then you could dose/medicate the fish as needed.

Hope this helps!


I checked the chemistry and the numbers came back nearly perfect - all 0s except for nitrate which was barely 0.5ppm. I researched around and did a 50% water change and I ended up giving the tetra a fairly diluted salt dip in a clean, separate bucket (1 gallon of tank water to 1 TBSP of sea salt) for three minutes. I've done this two days in a row now and the tetra is happy to swim and eat, and my wife and I both feel that the fungus has decreased. She noted that there was a small mark (scar?) on the side of the fish where the suspected fungus was but otherwise we feel like we're on the right path here.

We're in the process of picking up a small quarantine tank for any/all future purchases, too!


We've kept up the treatment and the little guy seems to be in good spirits, but it seems every week or so that the area that looked like it had fungus seems to return. His bottom fin seems to be about 60% gone, but he otherwise is happy to play, hide, and eat.

Can anyone think of anything that we might want to try or attempt here, beyond continued water changes and salt dips?

This is our first tank so I'm very hesitant to turn to over the counter medications (our diagnosis may be wrong, might ruin a freshly cycled tank, etc.) and have heard good things about the treatments that we've tried so far.


The disease went back and forth and I unfortunately had to euthanize the fish with a clove oil mixture last night. I feel quite guilty about everything that happened, and just wish we knew more before we blundered into all of this.

Next time around we'll have the hospital tank ready to go with an API Fin & Mouth powder (doxycycline?) that I'd seen positively reviewed online. Also feel **** that most of this probably came about because we added them way too early in the cycle, and I was overfeeding them and uneaten food was a haven for bacteria!

Rose of Sharon

So sorry for your loss!!!!

Don't beat yourself up over this. When first getting into the hobby, lots of people make mistakes. But now you know what you are doing, and I feel that you will be successful!!!! It really is a great hobby once you get everything started, and can really be a stress reliever from the world's craziness!!! :)


Thank you for your kind words. We've learned and gained some knowledge, but I still feel guilty that a living thing under my care died.

Do you happen to know if the illness/disease will remain floating freely in the tank, or if it only exists on/in the infected fish? I don't want to do the nuclear option and break down the tank and scrub everything, especially since the plants seem to be rooting well, and the cycle has finished.

I was thinking of letting the tank just happily roll along for a week or so (in hopes of letting the bacteria burn itself out?) before looking for hardier fish.

Rose of Sharon

That sounds like a good idea to let the tank go for a week or so without adding new fish. Is the loach still alive? If not, you may just have to make sure that your beneficial bacteria is still being fed to keep your cycle going.

I would think that the fish got the disease due to the cycling of the tank and the fact that they may have been genetically weak.

I am sure that other fish will do well in your set up. There are a ton of stocking suggestions for 20 gallons. You can do a search, and you will find a ton of ideas if you need some. You will also find info about how to add fish to a newly cycled tank. You might want to take your time and not add a lot at one time. Adding slowly is always safest.

Good luck, and again, please don't feel bad. You'll do better next time! :)

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