When we had goldfish back when I was a kid my mum went and bought this quite large snail from the local fish store. I think it cost a fair amount for a snail.
Anyway she got home and dropped it into the tank, and within about a second one goldfish came up and swallowed it whole.
I think sometimes the water flow in my tank is so strong my fish just sit 'downstream' waiting for food to come past them haha. I will try some spinach and we have some zucchini. They have sort of learned to nibble and pull on the plants so hopefully they should get the gist of how to eat it.