It doesn't look like fin rot, but looks more like someone is biting them. However, they have excess mucous on their tails and that could be a minor infection or water quality issue.
Check your water quality for ammonia, nitrite, nitrate and pH.
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As Juicebox suggested, do a 75% water change and gravel clean the substrate every day for a week or two.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Wipe the inside of the glass with a clean fish sponge.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use them.
Tip the bucket of dirty water on the garden/ lawn.
Add 2 heaped tablespoons of rock salt, sea salt, or swimming pool salt for every 20 litres of water. Keep the salt in there for 2 weeks.
If you do water changes, add salt to the buckets of new water before adding it to the tank so the salinity (salt level) remains constant.
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Monitor the fish and if there's no improvement after a couple of days of water changes and salt, post more pictures.