This is what put me off them.
Many years ago, when new to fish-keeping and totally reliant on the kindness of strangers, we accepted the advice of the Fish People who said a plecky would be okay for our two-foot tank.
He grew . . .and he grew . . . and he grew . . .
We bought a four-foot tank - and he STILL grew. He was enormous. In the memory of my mind's eye he was roughly the size of a small orca (and jumped as high!), though common sense tells me that that is unlikely - but I'll seriously bet he was comfortably over a foot long. Little fish started disappearing . . . he got bigger and uglier, and tbh I started to get slightly frightened of him. He was like something out of an Edgar Alan Poe story ("The Telltale Pleco" "A Cask of Amotipleco" "The Plecky of the Rue Morgue" - that sort of thing). He was sinister and mean and he stared at us all the time. I know that fish can't help but stare, what with having no eyelids, but it can be off-putting when that fish looks big enough to swallow the kids' guinea pigs without even having to chew.
I hated him with a PASSION! I wanted him to die, and he wouldn't - he just smugly sucked on the glass, with his big, horrible mouth, sneering at me. In the end I told my husband "Either that damn thing goes or I do," He thought about it longer than I felt was necessary, but eventually put him in a bucket and took him to someone he knew from work. I don't know what happened to him and I don't want to know.
However, it wouldn't surprise me if he is now 30 years old, and 18 feet long and has eaten a bullmastiff.