greens for fish

It's crazy that one small dandelion leaf like this is enough for all my fishes (paring knife for size). I have to go even smaller now, and offer just half a tiny leaf, minced up fine. The cherry barbs (now re-homed elsewhere) used to gobble it all up but now Fabio is the only one so eager. Samblu of course, gets none. I've noticed with the serpae tetras only the smallest ones really eat the greens I offer, the larger fishes kind of mouth it and spit it out. I end up siphoning out quite a few leftover bits on water change day that the shrimps and snails didn't find. So must offer even less. I wonder if it's because younger fishes seem to need more greens in their diet? I've heard this about another species- that if they're eating the aquarium plants, to offer more vegetable-based food, especially for younger, still-developing fish.