Fin rot?

Orange guppy losing it’s tail and seems to be fin rot. What should I do to help him recover?

Nothing fun about that

Do a 75% water change until the problem is resolved.
What size tank?
What tankmates?
What is your ammonia, nitrate, nitrite and GH?

5.5 gal
2 other male guppies
Ammonia is 0
Nitrates 0
Nitrite 3
Gh 120

Do a 75% water change and gravel clean the substrate every day for 1-2 weeks.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

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Add some 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 2 heaped tablespoon per 20 litres of water.

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 4 heaped tablespoons 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 using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.

If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.

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If there's no improvement after a few days with salt, post some more pictures.

I’ve put in aquarium salt but it doesn’t seem to have much impact.

Is anyone biting his tail?
Perhaps move him into another tank or a breeding net so none of the other fish can get at him.

Should I just keep doing daily water changes?

Yes. 75% daily. The aquarium salt is an astringent and will keep the tail clean.

Could it possibly be a fungus infection causing the fin rot?

I don’t see any fungus. The water changes and salt should do the trick. If you have any methylene blue, you can swab the tail for fungus to be safe. You can do this twice a day.

So I’ve caught my other guppy somewhat “harassing” the guppy with fin rot. Could that be a cause? Additionally, I’ve started applying Melafix to the tank. Was that a good move?

It is possible that one fish is biting the other fish's tail.

I wouldn't use Melafix but you can if you like. I prefer salt.