The oldest fish I currently have is a clown that I got when I bought my tank 3+ years ago. It was already full grown when I got it. I know 3 years isnt very old but this little guy is hardy. On the night I got the tank, I had all the fish that came in the deal sitting in a 20g tank while I was trying to get the 120g back up and running. I wasn't able to get it finished, so I decided to leave the fish in the 10g tank for the night. Well when I got up the next morning I found that the heater had failed. All of the fish,except for the clown, had been boiled. I dont know how it lived, but it did.