Koi.
Messy, inefficient, intolerant, plant-crushing, dim-eyed, personality-free, uninteresting, greedy, pond-greening inbred mutant horrorshow creatures.
In the sea we kill lionfish on site, but they are lovely and kinda cool... and taste brilliant.
However, the little 3-spot damselfish (Stegastes planifrons) is my nemesis.