It looks like an infection. The fish also appears to have the start of "Hole in the head disease"/ "head and lateral line disease". There are several pits on the face and head that shouldn't be there.
To treat the fin rot, try big daily water changes and salt (2 heaped tablespoons per 20 litres of water). If there's no improvement after a few days, use a broad spectrum fish medication that treats bacteria, fungus and protozoan infections. If it continues to get worse then you might need anti-biotics.
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Hole in the head disease is usually treated with Metronidazole. However, it is a minor case and the fin rot needs to be treated first.
You can sometimes stop hole in the head disease by doing big daily water changes and gravel cleaning the substrate every day for a week or two. And clean the filter and inside of the glass each week for a few weeks.
Basically try some big water changes and salt and see how it looks in a few days.
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What are you feeding the fish?
If you are feeding it mammal meats (beef heart and liver, etc), stop feeding that and use marine based meats like prawn, fish and squid. You can feed frozen foods like brineshrimp, daphnia, mysis shrimp, etc.
Don't use live Blackworms, live Tubifex or live Bloodworms.