Hi and welcome to the forum
The fish has clamped fins, which is normally caused by poor water quality.
If has flared gills, which is either poor water quality or gill flukes. Most likely water quality due to the water volume.
It has a couple of bare patches on its body where the scales are missing. One side looks a bit red, which could be blood.
------------------------
Do a 75-80% water change and gravel clean the substrate every day. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
-----------
Feed the fish 3 times a week and remove uneaten food after feeding.
-----------
Get a bigger home for the fish. The bigger the container, the more water it will hold and the healthier the fish will be. You can use plastic storage containers, large Tupperware containers, even plastic buckets. Just make sure they are free of chemicals and have not had anything poisonous in them.
-----------
Try to get a filter on a bigger tank.
Add some live aquarium plants to help keep the water cleaner. Duckweed and Water Sprite are both floating plants that help keep the water clean.
-----------
Add some salt to the tank.
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 level tablespoon per 5 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 level tablespoons of salt per 5 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur.
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.
------------------------
This issue is caused by poor water due to a small volume of water, lack of filtration and lots of fish food being put in the tank. Big daily water changes and a bigger tank should help.
