First thing, let me say that I rarely comment on disease issues aside from offering general advice. I have been fortunate in my 30 years in the hobby never to have had more than a couple of issues, and I was/am lucky to be able to turn to professional biologists/microbiologists I know online for advice. Prevention is always preferable to reacting once a problem presents itself, so my comments following are in the scope of preventing and why some things can lead to disease.
Stress is directly responsible for about 95% of all fish disease issues. So avoiding stress, or keeping it minimal, goes a long way to healthy fish as they are then better able to fight of this and that. Stress occurs from many things, from water parameters, aquascape, numbers of a species, other species, lighting, water currents--any environmental factor in fact that is not what the fish "expects" will cause stress.
The frayed fins is without much doubt due to fish eating them. You have Serpae Tetras, and this is a species notorious for fin nipping. It should never be combined with sedate fish like angelfish. Now I realize this tank has been running for six years...but that does not mean it has been "working" with respect to the fish. Only a fish living in the tank could tell us this, and that being impossible (until it becomes so severe they die) we can only rely on our research and understanding of species. And never putting these two species together is a significant issue.
Related is the pictus; this fish is active at night, and this is stressful to angelfish and many others for that matter. Pictus is also a shoaling species, and needs a group of five (or more). Peaceful but it is predatory and as it matures it will eat small fish. Should be kept in a small group of at least 5; single fish may pine away. Tankmates should not include sedate fish like angels, discus, gourami and even cichlids as these will be pestered by the nocturnal habits of this catfish, nor nippy fish like barbs.
It is logical to assume that the stress from just the above over time wore down the angelfish, weakening it so it became more susceptible to pathogens. The minimal water changes also affect all of this. If nitrates rise at all, it is a sign that water changes are probably inadequate, or combined with other organic issues (substrate, filter cleaning, fish load, fish feeding). My tanks are in the 0-5 ppm range for nitrate, and have been for over ten years now. I don't test regularly now (I did, every week on the day prior to the weekly water change, for several months, then only spot checks), because I don't have to; my routine has not changed in all these years, and I know that the nitrates are this low and also the pH remains the same in the respective tank. That is stability.
Fish release pheromones and allomones, chemical signals other fish read. Even if there is no visible physical interaction, the fish may well be "thinking" it, and that is high stress. Another reason for more substantial water changes is to reduce these substances, something no filter can achieve.
Hope this helps.