Hi and welcome to the forum
The fish has dislocated part of its bottom jaw. Sometimes it goes back in by itself, other times it doesn't and the fish dies.
The fish is also covered in excess mucous (creamy white film over the body and fins). This is caused by something in the water irritating the fish. Doing a 75% water change and gravel clean the substrate every day for a week should fix this.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
The fish looks like it has a microsporidian infection (the muscle tissue under the skin is white). This is caused by the fish eating a fish or shrimp that had a microsporidian infection. Adding salt to the tank for 2-4 weeks should get rid of it. See below for directions on using salt.
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SALT
You can add rock salt (often sold as aquarium salt) or swimming pool salt to the aquarium at the dose rate of 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, Bettas & 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 (4 heaped tablespoons per 20 litres) will affect some plants and some snails. The lower dose rate (2 heaped tablespoons per 20 litres) will not affect fish, plants, shrimp or snails.
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.
When 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.
When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.
