Hardest fish to keep & what is your oldest fish

[Trigger Man]

Copperband butterfly and Regal Angel fish were the hardest ones for me to try and in the end both slowly died away despite my best efforts.
The Achilles tang was the one that still has me confused, as it was great for over a month never showed ick, ate everything, had great color and then one day just died in a span of 5 hours, breathing heavy and slowly did the sideways thing, but no other fish or coral in the tank looked un normal and the water checked out perfectly when I tested it.
My longest living fish are currently a little over 2 years old (were introduced within 2 months of eachother) and are my Crosshatch trigger, Jumbo Chromis, Purcula Clownfish.