Can I be sued for slander over my opinion on a forum?

I run a small forum on the subject of UFO's and the Paranormal.from time to time certain subjects can come up that get rather heated and people and organizations can be called out by name.Some times peoples opinions can be come rather strong(including mine) but i don't let it get to the point were its absolutely vulgar.Recently i was told that i can be sued for slander over my opinions and the opinions of others on my forum. Is this true?