My experience with them was not as bad as I keep seeing here. I have touched them, had one in my hand and did not feel any sting. I had one in my small 12 gallon nano. My book states that some species can pack a powerful sting that can burn or kill fishies, giant clams and other. Again, I didn't feel it from mine. Also, they need to be fed on small planktonic foods. Their tentacles are large, but they are quite delicate and feeding large foods may damage them.
Other anemones sting also. I believe (based on all the info I have read) that you would have a greater chance of a RBTA or GBTA eating one of your aquarium fishies than a tube anemone.