Regular white vinegar is what we used at the zoo to clean glass.
On the subject of your big tank. What is the caiman housed in currently? My experience is that small crocodilians are often overwhelmed by such a large enclosure. Even with lots of places to hide it is just to much. What you could do is block off a smaller section of the cage and then increase the cage size as the caiman gets bigger. Just my opinion. D.C.