They are just "for" reptiles. Although they do hold water they are not designed for it. The glass is much thinner and cracks easily. They are not made to hold the pressure and the weight of water along with all the other variables in an aquarium.
Yes, they "can" be used as aquariums, but that not what they are "for". To be quite honest, it's not a very good idea. 40 gallons of water in a floor could be disastrous.
I used to be in the reptile business as some here already know. I have cracked countless 40 breeders just from the slightest things. Like bumping the side with my elbow while cleaning for example. If you do have one that the manufacturer specifically states that can be used as an aquarium then it is highly unlikely you have a true 40 breeder.
