Spawning carpets, spawning coral. Figure eight puffers, cowfish and boxfish.What single organism can wipe out an entire tank by virtue of it's existence? I don't know of any, but I've only been in the hobby 4 years and there is a lot that I don't know. Most of the "pests" people describe are just that, pests. Nuisances. Not all that dangerous and certainly not capable of single-handedly wiping out a tank.
That was an honest question for anyone who can answer it.
Alas these don't really come in on live rock
Majano and Aptasia can destroy coral, they are incredibly prolific and they can sting when they get big enough. If left unchecked they can nuke a tank, but it takes a while so you have plenty of time to deal with them. Dinos if not controlled will cover everything and choke it out the light, also there is a toxin in the dinos that will kill herbivores. So they can wipe everything out but fish, but once again you have plenty of time to deal with them. Hell even any macroalgae species can choke out everything. There is a reason you don't see the various macroalgae species in the hobby.
There are lots of things that can nuke tanks, but the answers as to what to do are out there.
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