I know, the net is a wondrous source of confusion sometimes. Anyone can say anything and you have little way to verify it all.
But for what it's worth, because fancy goldfish haven't really got a stomach as we think of one, they do better with frequent smaller meals. Most of them are pigs, and eat too much if allowed to. But their gut is meant to function best when they graze. They can't graze in the tank, so the best you can approximate it with is more frequent meals.
I don't suggest you stay home just to feed fish, but a pinch in the morning, one sometime in the afternoon, one before bed, that should help, I hope. Peas are used often to manage constipation and they work, so if that happens, go with peas.
Growths on the fins can, I only say can, be related to water quality problems, as can split fins sometimes. It can be opportunistic organisms that are always in tank water, taking advantage of some weakness, whatever it is.
I often suspect inbreeding, because it is rampant among commercially bred fishes of all kinds, and the effects are not always readily visible defects.
It can be so frustrating to have a problem continually recur and not be able to manage it. But clean water, well cycled filter, enough but not too much food, these things are manageable, and sometimes they are all you can do.
I quickly gave up using any commercial filter inserts and always use my own media in any filter. Even the smallest of the small will fit a bit of sponge if nothing else, and that way I always know what's in them. I don't think carbon is good to use all the time, but it is useful if there are contaminants in the water it can adsorb, which is what it does. But it does get 'full' and recharging it is not practical for most, though it is possible if you have access to the right equipment.
I don't think filters become 'infected' as such, unless you have ick or some other parasite or an outbreak of something like columnaris or neon tetra disease, which are due to known organisms. Then I think sanitizing a filter might be helpful. The biofilm that accumulates on all surfaces exposed to the water is not, to my knowledge, harmful to any fish. Many fish need it, and do very poorly without it. I've never seen any thick buildup inside my filters, only the guck at the bottom where the water first enters, which is soon squeezed out when I do the sponges.
I have never kept discus, so I really can't comment on the remarks about how they are kept, but the only thing I get that infests my filters has been algae, and now not much, thanks to the foil. I did use mylar film, but it tears too easily and then the algae reappears.
But there many organisms in tank water at all times that don't cause issues about 99% of the time, unless there is a weakness for them to exploit. Hence the term opportunistic infections. One good article I read recently pointed to ammonia as the number one reason for 'mystery' deaths and disease in most fish, because many tanks have chronic low levels that do damage over time, either due to lack of maintenance or improper maintenance. But I doubt that's your problem, you've been taking good care of the tank. Sometimes we just don't get answers.
Wish you luck. I have an elderly cat that has become anorexic after having a stroke and she makes me crazy when she refuses to eat. I have been tearing my hair out worrying over her.. I truly understand how frustrating this kind of chronic problem can be.