if you have LR in your tank... it can smother the good stuff that lives on it... the microogansims and coraline algae and what-not.
I had it in my 75 when I first set it up. My LFS sold me penecilin (I think... it was a few years ago). 1 tablet a day for 3 days wiped it out... no problems since, and no adverse side effects to my tank either.