long term gradual demise of schooling fish in a healthy tank???

[Magnum Man34 148]

so, in my south American tetra tank, most fish are thriving, however my schools of cardinals, and rummy noses, slowly grow smaller, and have to be restocked once a year or so, they aren’t noticeably dying off, but gradually one by one, yet the other fish populations remain stable, and on my cardinals I have one old timer who is larger, and lived through 2 schools of gradual die offs… I love both of these species , and can’t figure out why the schools can’t be maintained… this last group of rummy’s has been lasting longer, but the cardinals have been dying off more rapidly…
my goal here, was as I retired, was to slow down on ordering fish, but seems if I want to maintain a species like the cardinals, I’m going to need to resupply every 10 to 12 months, and it’s strange I have the one, that had been in this tank for around 4 years…
thoughts???

 

[Magnum Man34 148]

old timer ( middle of this picture ) still doing well, while there are only 4 left of the dozen, that I added 5-6 months ago…

Google was no help…

Google Search

 

[TwoTankAmin28]

I had something similar with recline barbs. I had a really big one which was the lone survivor from a previous group. I put in a new group and it was the Alpha. It was in the tank when I got the eggs and fry, He died this summer along with one of the offspring that had survived and grown to a decent size.

One of things I have learned over the years is that people and tropical fish have a lot in common. We both can and do die from heart attacks and strokes. Some of us die young while other live well beyond expectations. had an aunt who die at 101. And I have a few fish that lasted in the way you describe.

(edited to put the d in redline and a space in Ihave)