I agree with the above but wanted to add one caution....
Dandylion leaves are, indeed, a wonderful treat - and the fish act like I have dropped candy in the tank when I feed them. BUT..... just rinsing a dandylion that has been "treated" in a lawn is not enough. Most weed preventatives or weed killers are systemic - made so that the plant absorbs the poison into its stems/leaves/flowers and then dies when it has too much. That means that the leaves of a dandylion you pick from a lawn may have incorporated poisons in it - ones that you cannot just wash off - they are inside the leaf - part of the plant.
Only feed dandylions (and other outside greens) that you
know have not been sprayed or treated in any fashion. I have a small area in my garden, tucked behind the hydranga bush, where I have 3 extremely LARGE, healthy dandylion plants that I use for all the leaves I harvest. I put them, blanched, into the tank on a clip, or I blanch them and add them to my gel food, along with broccoli, spinach, peas, etc....