The Carpet Anemone is a fairly difficult specie to keep. It requires good water movement and fairly strong lighting, in your case with a 70-gallon I would say 250 watts of metal halide.
They grow rather large up to 2 ½ feet in diameter. Their sting is rather potent, will harm other corals and can cause severe reaction in some humans. As it grows it will dine on small fish and inverts.