I would think that alligator dropping would feed a lot of microscopic life, which the guppies might feed upon. There might also be scraps from what the alligators ate.

A strain of Daphnia has been circulating in the American hobby for some decades. Livebearer guru James Langhammer picked them up from an alligator tank in the Moscow zoo. I recently picked up a few  and their care is pretty much like the "local" Daphnia.

It is interesting what may be "incidental" creatures may be in an exhibit. I do wonder how they change the allegator's water without doing the guppies harm.

Do most European zoos have an aquatic section? Many American cities have aquariums which are separate from their zoos. There is still overlap in what is kept. Other cities include an aquatic section within the same facility. And I think that almost all of them belong to the same professional association.

Thank you for sharing about that zoo and their guppies!