Mostly it's a matter of your fish getting used to you, and it varies with the fish. Every fish I've owned has eventually accepted hand feeding (not that it was necessary, but I just enjoy doing it). I'll usually start by just having my hand in the tank when I feed. Once they seem comfortable with that, I'll try holding a bit of food and letting it go when the fish gets close. Eventually, they should start taking it right from your hands. My newest fish (Meiacanthus mossambicus. . .a.k.a. Harptail Blenny) was eating from my hand after the first week. My maroon clownfish would actually let me pet her. My Purple firefish goby, on the other hand, has just finally started coming out of his hiding spot with me near the tank.