My ocellaris clownfish "pair" is probably the longest "couple of fish" I've had at this point, since 1998, had them from the beginning .... the only problem is, they are not the original male and female anymore. The current female was once the male, but his female jumped out of their tank .. in .. um .. 2001 or 2002. So in fact they might still be the oldest fish I have anyhow as the next oldest fish, who may be my longest individual, is my desjardinin tang whom I bought in 2001 if I recall correctly. If not 2001 then early 2002. And for some reason I'm thinking my current female might have been a replacement male but I can't for the life of me remember what might have happened to transpire to need a new male, ie. whether it was the original male lost or the original female. It's been that long, apparently.
Up until about a year ago (I think? Maybe a bit longer now?) my oldest fish had been my sixline wrasse whom I had also had since the late 90's. Unfortunately though he found the magical portal to the reef in the sky.