I would use sponges instead of filter pads/ cartridges. You can buy sponges for different brands of filter and cut them to size with a pair of scissors. I use AquaClear sponges but there are plenty of other brands. Filter pads are a sales ploy by companies to get you to keep buying their products. The filter pads are usually poor quality and don't last long, whereas a sponge will last 10+ years and simply get squeezed out in a bucket of tank water and re-used.
When you replace the filter pads you get rid of the beneficial filter bacteria that keep ammonia and nitrite at 0, and your fish suffer because of that. It then takes another month or so for the bacteria to develop on the new pads and when they get replaced, you start all over again. It means you have constant ammonia and nitrite problems in the water and the fish suffer. This could be why your fish is still having trouble with his fins.
I would look for some sponges and cut them to fit in the filter.
If the current filter pads/ cartridges are new (less than 1 week old), remove them completely and put sponges in their place. If the cartridges have been in the filter for more than 2 weeks, I would add some sponges to the filter and leave the pad in there. Wait a month or so and then remove the cartridge and put more sponge in the filter.
You can get round/ cylindrical sponges for some brands of internal power filter. These sponges have a hole through the centre and can be put over the intake strainer of most external power filters. They trap gunk and also hold beneficial filter bacteria and can be used in conjunction with other sponges and cartridges in the main part of the filter.
Do not add any ammonia adsorbing granules. Just keep doing water changes and let the filter develop the good bacteria. Hopefully in about a month's time, the sponges should be full of good bacteria and the fish will be able to carry on with its life.
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Filters should be cleaned at least once a month and every 2 weeks is better. However, do not clean a filter for the first 6 (preferably 8) weeks otherwise you can wash off the beneficial bacteria and start the cycling process over again.
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You should check your tap water for ammonia, nitrite and nitrate. Sometimes well water can become contaminated and cause problems to fish and anything that drinks it.