It is not my secret, it was the Otto's secret. I never found out. The other nine did not live many years.
I haven't kept a lot of fish as my Clown Loaches have dominated my fishkeeping hobby and they are 30 years of age now.
I did have an Arnold’s Red-eyed Tetra (Arnoldichthys spilopterus) male that lived for 16 years and 5 months.
I had a Hillstream Loach (Gastromyzon borneensis) for 12 years.
My Glass Catfish shoal lived for 11 years.
I would say I keep fish in a simple, basic and inexpensive way using outmoded technology such as undergravel filters and outmoded practices such as lower stocking rates but I maintain them well and consistently and never lapse. I try to eliminate risks where possible. It is something I have a knack for and I have been lucky.