there are a lot of solitary fish in the world, so we can't think of them like humans being locked in a room alone. Many, many fish only see others of their kind once a year to spawn. Kept in the consistent conditions of aquaria, many fish never feel the urge to spawn.
This would be a single fish to occupy a 50g breeder tank once I have sold off all of my young bristlenose. It will be open top and I thought it would be cool to have a large fish to interact with (ie handfeed). I've had goldfish this way and thought I"d try something different. The tank doesn't have to look cool (it'll be in the office and out of sight to all but me, but it would be nice if the fish did. \
A big fish would be fine (great, really) as long as it wouldn't outgrow the tank, or grow to the point where it would not be comfortable. I think up to 10-12" would work in a tank with an 18" smallest dimension, right? I guess a pair would be ok, too, but I"d really rather have just one.
I thought that an aggressive cichlid would be the best candidate for a solo existance but maybe I"m wrong. I'm new to that sort of thing and only familliar with fancy goldfish and community fish.