When I had my Jardini, I would have to quickly put my hand in the tank when he was at the opposite side when I was moving stuff around and cleaning up, ect. If not, right when my finger tips touched the surface of the water, he would hit my hand with max force. Drew blood a couple times, nothing major, just scares you a bit when you're not expecting it, haha.
They just have to know that your fingers aren't food, so just be smart about how you put your hand in the tank and you shouldn't have to worry really. And they're not going to rip a finger off or anything, haha.
Edit:
Mine was about 15 inches when this started. Before then, he was a pussy cat. About then he started killing off tank mates too. =/