If the glass thickness is the same as an aquarium, it's probly an aquarium that they added a screen lid to. Tanks made for reptiles usually use thinner glass. If it's the exact dimensions of a 20 long, shows no difference in the actual structure of the tank, yer probly fine.
Tanks made specifically for reptiles usually have different doors and other modifications appropriate for reptiles.
Also, if it were not designed to hold water you'd probably have water on the floor by now!