It's the first time I hear from such a problem...
Is the water line correct?
Just a glance at the water line level may tell you there's a problem with it or not. Check for le color change location.
If the water line is correct, then...
What's the model's volume?
Since its displacement is 1.5 tons, does the volume fits such a datum?
In the metric system, a displacement of 1.5 tons means a volume of nearly 1.5 cube meter. Check it broadly in the imperial system.
If you find an inconsistancy, then...
Did't you change the water density datum?
Broadly, in the metric system, fresh water has a density of 1 Kilogram/liter, ocean water 1.015 and the mediterrannean 1.033 (as far as I remember
). Check for your water density, convert it in metric system and make sure you get a consistent number.
I grew up with the metric system, and trust me: Imperial system %ยงยค# !