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.
Here's my article on their removal.