To add my 2 cents..
Zuchinni is the best in my mind. We grow it our selves in the summer. But, dont let it grow to big or the seeds will make a mess in the tank. When it is smaller... cucumber size, no mess.
Hard to find this time of the year though. All I see is zucchini squash... and it tends to cloud my tank.
Havent tried sweet potato, but soon will.
I see no reason to blanch or cook any of it. Just more time. Use a clip, my preference. Using the fork method works too, but then you have to fish it out of the bottom of the tank.