I think the hardest fish to take care of was my fan tailed goldfish, I killed like eighteen of them in six days. They produced way to much ammonia for the system I had and I didn't know it. So, I can't keep goldfish to save my life. The easiest fish I had were angelfish and sharks, which lasted for a couple years. The second one I had good luck with was beta fish. They were surprisingly hardy despite attacking the window every time someone or something approached. So what was the hardest and easiest fish you've had as pets?