When I did my first Saltwater tank, I thought it would be a nice decoration to add a 'Starfish' my mom brought back from Key West. Anyways, I didn't know it was an actual living organism that was dried out. About 2 weeks later the star fish turned to moosh, and the entire basement wreaked. Cycled my tank good though
Too bad I had bad advice about SW husbandry, so it didnt matter.