I have a large algae water that's getting to big for my tank... Could I donate him to the zoo?

The creature conservatory in Ann Arbor mi takes donations like this. Contact them and they can help you out.

Formerly employed at a zoo, contact guest services and ask to be put into contact with the curator or assistant curator of the aquarium, no promises but this is not the type of question that the general public could answer. At the very least the AC would be able to better direct you to an appropriate destination for your specimen. Much of the zoo's collection especially in the area of aquatics are not usually on display that have quarantine and hospital tanks for incoming animals.

Probably not.

It's unlikely they accept donations like this simply for cross contamination of possible diseases