Betta fin rot

Hello! So I went away for a while on a trip and while I was gone my sister seems to have forgotten that fish aren’t just toys and completely avoided doing any water changes. I did a major one immediately once I got home (3 gallon tank with nerite snail) and noticed some black on his fins. Now the tips of his fins are normally white so it especially stood out. On one part it didn’t look too bad but on the other it looked decently obvious.

I’m nearly 100% sure it’s fin rot and aside from planning to do a minimum of 50% water changes daily for however long it takes, I need to know if it needs treatment. It looks to be moderately bad, so I think using aquarium salt would be sufficient. I have a good idea of how to use it but before I start, I want to know about it’s impact. Since nerite snails come from salty water (I think) can I still use the salt in this tank without having to remove him? I don’t currently have any other tanks so I’m not sure what I would do if I had to use aquarium salt and he couldn’t be in there.

Please let me know how to proceed from here. Thanks!

The snail will be fine with salt.

You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.

If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.

After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.

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Do a 75% water change and gravel clean the substrate each day for 2 weeks. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. Wash filter materials in a bucket of tank water.

Add salt to the new water before adding it to the tank. Basically get a bucket and fill it with dechlorinator and tap water. Aerate it for at least 5 (preferably 30) minutes. Add salt to that bucket of water. Aerate for at least 5 more minutes (or until salt is completely dissolved, and then add that water to the tank.

You add enough salt to treat the bucket of water.
eg: you have a 10 litre bucket, so you add 1-2 level tablespoons of salt per bucket.