Zuchinni in it's self shouldn't pose a problem unless it's left in the tank, or you put an ungodly amount in a tank.
Every week at the lfs I work at we feed zuchini to the african cichlids, plecos, barbs, and other plant eating fish.
We just take a zuchini and cut it into slices the long way, and put it in the tank raw. The next moring we take it out. It's not in the tank for more than 24 hours.
Never have we ever had a problem with it.
The only thing that could happen, and has happened with my 40 gallon at my house, is taht my africans had never eaten zuchini before, so they were'nt touching it. I left it in the tank, and after 24 hours, it started to turn all mushy and fall apart. This makes the tnak cloudy, but shouldn't kill fish.
Rotting zuchini shouldn't preoduce enough ammonia to kill anything.
65 gallon low tech planted