I have read before that undergravel filters should not be used for saltwater tanks. is this true? I have a friend here at work who is new to the hobby and is setting up a new tank for fish & corals. today I was talking to him about his set up and he mentioned that the lfs has sold him an undergravel filter and said that this would help as well as the sold him on crushed coral. 
. any information will be appreciated.