Is the base 2500 x 1000 in 10mm?
If that is the case then the height wont matter to much becuase the base will probably crack. Even at a safety factor of 2.5, which is not ideal when you are talking about 1.5 tons of water, you should have 14mm base. Ideally you should be aiming for a safety of 3.8 so 16.5mm base.
If you are planning a 5mtr x 1mtr tank then a 20mm base or a double layered 10mm glued together and well braced would be a safer option.