It's either an infection in the brain, or the fish has brain damage, or it was born with a neurological problem, but that should have shown up as soon as you got it.
The fish also looks skinny.
These problems are normally caused by a protozoan or bacterial infection in the brain and occurs in dirty tanks or tanks with a lot of rotting organic matter. Fish can also be infected in ponds or at shops before you get them.
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Wipe the inside of the glass down with a clean fish sponge.
Try doing a 75% water change every day for 2 weeks. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter. Wash filter media/ material in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.
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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.
