-is to take your iguana's body to the vet and have a necropsy (autopsy for animals) done. It will put your mind at ease, help you decide what to do for your other igs, and further vet knowledge of why iguanas die. It could help someone else in the future.
Were you using water purifier/softener/ whatever for some kind of fountain?
I am so sorry to hear about your loss. I hope your other igs are doing ok.