I think from whats been said I'd agree with people that it looks like ammonia poisoning over the course of a few days. Even at low levels .5 or .25 it can kill fish. Really sorry to hear you had some of these for a long time too.
I had a really big disease outbreak in one of my tanks quite a few years ago and lost a lot of fish really quickly and it is a punch to the gut when it happens infront of you.
Like above I'd recommend getting an API liquid test kit they are the ones most people go by so have a level of trust that is worth paying attention to.
I am curious as to what could have caused the ammonia in the tank though, I think if we get to the bottom of that you will have more success in the future. So if you don't mind a few more questions
How often to do you clean your filter? And is it internal or external?
Do you have a gravel substrate and how deep is it and how much do you clean it? A
re any of your plants doing badly and do you know if you have any terrestrial plants in there?
When you feed them is there ever food left over and if so do you remove it or leave it in there?
You mentioned about some difficult times when tank maintenance drops off (it happens to us all, pretty unrealistic to think otherwise) but when things are more settled what is your maintenance routine? Do you do weekly water changes etc? And when you do a water change how much do you each time?
Last thing to mention - might be worth just putting some food in the tank to rot now and feed the filter until new livestock is added.
Wills