As Ro did like the zucchini! It sank though and she wouldn't go down to eat it, so I'm not sure how much she actually got.
When I give my goldfish veggies, I make sure they don't sink by stringing them to a suction cup on the side wall of the tank. I run a plastic zip tie either through or around the food and the ring on the suction cup. (It is originally for air tubing, so has a ring to put the tubing through.)
If the food is something thick, I will punch the zip tie through it and loosely dangle it. If it is something more loose, like spinach, I will roll it and pull it tight.
I used to do it with fishing line, but found a pack of several hundred food safe mini zip ties on sale for almost nothing and they have made life easier.
i usually leave the food in the tank overnight (mine forage like crazy while I am sleeping...) and take out whatever is leftover in the morning. Then I cut the zip tie.
My goldfish don't like zucchini very much, but they devour broccoli.
I blanch it and dangle it by the stem... They go crazy picking the little leaves (?) off the top.