Both my african knife and former BGK used to readily eat frozen shrimp from my fingers. First I started off be regularly feeding market shrimp till it was their favourite food. I then started holding small pieces in my fingers really close to their hiding spots to feed them. Over time I gradually coaxing them further and further out before allowing them to grab the shrimp. Now I can just stick the tips of my fingers in the water and they swim right up the surface to hand feed - makes for a great show for guests.