The biggest difference is that a FISH tank is taller, where a reptile tank is shorter and has a greater floor area. This is done for two reasons: It's easier to heat the bottom of the tank with heat lamps on a shorter tank. The reptile has more running around (slithering around?) room with the greater floor area.
The fish needs neither floor space (it swims in three dimensions) nor the need to heat the floor. So, fish tanks are taller, rather than wider, so that the owner doesn't take up valuable floor space in his/her home.
A few more things to look at is glass thickness. Reptile tank will be less thick. Also sealant on fish tank will be slim and not messy while reptile tank will be broad and messy. Also in reptile tank on the underside or somewhere it might say do not fill.