To the best of my knowledge (which isn't much), chams can only be kept in aquariums as hatchlings, requiring more ventilation and hence needing some type of screen or mesh enclosure, when they are older. My personal preference is for the screened cages (can be found online at any number of sites, look around for best price, or try eBay - tho they my not be the cheapest. Also, there is one site that makes custom cages with a larger mesh screen, which "helps prevent lost toes & claws", but for the life of me I cnnot remeber what it is, I think if you do a search for "custom chameleon cages" you can find it, the guy is based out of Margate, FL). They tend to keep the food from escaping, and it's a little more sturdy - I keep a lot of lights, etc on the top of the cage, and I have a feeling the reptarium would have caved by now...
Anyway, I had a point with all this when I started... Oh, yes, how long can you keep a cham in a tank... I would say 3 months is pushing it (someone correct me if I am wrong). Their cage tends to get stagnant w/o proper air circulation, and they need the room to move about.
Hope that helped. Good luck with your chams!
~C