The zucchini sinks after freezing, or after soaking for a while. I've tried weighting it down with a rock with poor results. You can purchase veggie clips--a plastic clip on a suction cup, or weighted so it sinks. Some people will use a tank-safe fork, also.
With veggies, I think people give up to quick. Most fish take a few tries before accepting something as food, so be patient. Don't let it rot, but leave the veggies in there for 2-3 days the first few times.
For other food items, if you have problems getting them to the bottom of the tank throug the top and mid-water feeders, use a riser tube. Cheap, and you can pour the food directly to a specific location. This worked great for feeding a peacock eel I had--he learned what the tube meant, and would swim up it in search of food.