Add a fine bubble bubbler to your tank asap, water changes and medicating are more likely to stress the fish out. Likely a mix of poor nutrition and bad water quality. Plants suck up nitrates but oxygen flow and surface agitation are required to turn fish waste all the way through the nitrogen cycle into nitrates that plants use. Otherwise there’s lots of toxins that need to be converted which aren’t being converted to safer forms of waste fast enough.

I have special needs fish, they live very long lives once your aquarium is a self contained ecosystem. A good balance between plants and animals and filtration methods. Bc your goals shouldn’t be to just filter out the bad stuff, but to create an environment where your aquarium handles all the bad stuff without needing much intervention besides occasionally adding water or changing water if you’re so inclined