What sort of filter is on the tank?
How often and how do you clean the filter?
What is the ammonia, nitrite, nitrate and pH (in numbers) of the tank water?
What else do you feed the fish besides the pellets?
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Don't worry too much about him starving because he is in good shape and a reasonable size and can live for a few months without food. However, it is not normal for fish to stop eating. The most common cause of this is poor water quality and an infection in the mouth or throat.
I would do water changes more often and do bigger water changes. Small water changes once a month allow harmful disease organisms to build up and gradually they affect the fish.
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Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. Then do it once a week after that. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
Add some salt, (see directions below).
If there's no improvement after a week of water changes and salt, post another video of him eating. Upload the video to YouTube and copy & paste the link here. We can go to YouTube to view it.
If you use a mobile phone to film the video, have the phone horizontal so the image fills the entire screen and we can see the entire fish.
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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.
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, 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.