Fresh zucchini isn't messy at all. It'll last for days before it starts to fall apart (not that you'd wanna leave it in there that long).
I usually blanch my zucchini in boiling water for, like, 30 seconds, and then put it in cold water to stop the cooking (and to cool it off before I give it to the fish). I've tried microwaving, but it's so easy to overcook it that way. Once I put an overcooked piece in the tank, and when I went to take it out the next day, it just dissolved into zucchini mush in my hand. --It was gross
But very lightly cooked zucchini is good for at least 24 hours. I usually drop mine in at night and take it out in the morning. Spinach and kale are good too, and not messy.