Red Planaria flatworms
When I bought my 55g tank, it was infested with Flatworms - Red Planaria - which propagate via fission. They are known to increase rapidly, smothering the sandbed and corals. If they all die at once, that crash will wipe out a tank due to the toxicity of the flatworm juice. I vacuumed out 1000's over 10 months, and then dosed my tank with Salifert's Flatworm eXit which removed them entirely. For a time being, some of the local fish stores I visited had these infesting their coral tanks and that left me little reason to buy anything new. I did bring it to their attention so they remove these and stop spreading this pest to other aquariums, and over time the stores got rid of the problem.
Here's my article on their removal.