I had a 6 inch Bobbit Worm...
I thought he looked neat...and I threw him in the tank...until I learned that he can eat fish, and use bones to line his lair!!
I quickly took him out (in 2 pieces). I have recently seen a small on in the tank now too...but I have been unable to catch him.
90 Gallon Fresh Water Planted w/ CO2
7 Brilliant Rasbora, 6 Harlequin Rasboras, 3 Dwarf Neon Rainbows, 1 Red Tailed Shark, 2 American Flag Fish, 4 Skunk Botia Loaches
40 Gallon Breeder Salt Water: 4 inch DSB
(2/3 Agaramax & 1/3 Sea Flor), Bak Pak 2R Skimmer w/ BBX, 2 x JBJ PHs, Coralife Lunar Aqualight (2x96w), 30lb Base Rock and 40lbs of Live Rock.
Inhabitants: Purple Firefish, Pygmy Angel, Lawmower Blenny, 2 Cleaner Shrimp, 1 Peppermint Shrimp, Pistol Shrimp(HH), 1 Emerald Crabs, 3 Porcelin Crabs(HH), varying amounts and varieties of hermit and snails.
3 Gallon Eclipse:
1 Goldfish (won free at a fair...cost $50 in equipment)