That thing is Medusa Plecu, a Brazilian catfish, and at first glance it looks like Cthulhu in the flesh: pitch-black, ugly as can be, and a whole mess of tentacles where a mouth probably should've been. Hell, it even looks angry! We have awoken the Dark One, and he ... cleans our fish tanks.
Well. Winning the war was easier than we thought.
In its natural habitat, the Medusa uses its decidedly non-evil tentacles as feelers to forage for food. Thus, it's a natural transition to baby's first bottom-feeding, cuddly algae slurper. This mythos is getting worse by the second.
Jeremy B. via cichlids.com
That is not dead which can on the bottom lie, and with strange tentacles even algae may die.
Shockingly, Medusa Plecu don't like each other. Sticking two of them together in the same tank can provoke an all-out, um, mild tussle for the ages. So ... don't do it? Or maybe you should; it might be more fun. Nobody tell Cthulhu's cult about this, OK? It would severely bum them out.