I've also had loads of problems with Green Iguanas, but it seems to be a bug and have nothing to do with food quality (not even sure you can change it for exibits). Every time I get a notice that one of them is hungry/thirsty/starving, it's because their exibit has become "invisible" to the keepers. The call keeper button doesn't work at all and the keepers totaly ignore this exibit. Followed the keepers for a while when I had this problem for the first time, and they went to the other nearby exibits regurarly, but totaly ignored this one (and yes, all of them, including the affected green iguana exibit was in the same workzone).
Targeting the exibit and clicking the "move" button and then exit (not actually moving the exibit) is a temporary and in some cases, permanent fix (you know that it works if you can use the "call keeper" button after the "fake move").
In one of my zoo's I eventually just got rid of that exibit and the Iguanas as the problem kept coming up and I got protestors everytime I went afk cus of this. But in the zoo I'm working on right now I had this problem just once, and after I did the "fake move" of the exibit I haven't encountered the problem again.
