A lot of people blanch the veggies before adding them to the tank (pour boiling water over them or submerge them in boiling water for 30 seconds or so) to soften them a bit, but I never do. I just wash them. For zuchinni or cucumber, I usually slice a little 'canoe' and scoop out the seeds. I have 'veggie clips' (plastic clips on suction cups) that I use to keep the food stable and submerged, but you could also wedge them under a rock or piece slate. For mid and upper level fishes, you can sometimes float leafy veggies (I do this with carrot greens) and catch the stem in the canopy lid.