You most probably find that all things that make your fish sick are already in your tank, and like us humans, it usually takes other external factors to fall prone to these, like that **** job I got (stress). I put heaps of crap I collect from the ocean, sand, water, urchins, bit's and bobs, little fish. If anything I think it diversifies my environment. However, you need to properly id the star, they can play havoc.