Rectangular tanks water capacity

There are three rectangular tanks in a building. The length, width and height of the first tank are $m\text{ meters}$ each, and the length, width and height of the second tank are $n\text{ meters}$ each. However, the length, width and height of the third tank are $m\text{ meters}$, $n\text{ meters}$ and $1\text{ meter}$, respectively. Initially, the first tank is full of water, while the second and the third are empty. When the second and the third tanks are completely filled with water transferred from the first tank, $85,000\text{ liters}$ of water is still left in the first tank.

If both $m$ and $n$ are positive integers, what is the value of $m$? ($1\text{ meter}^3 = 1000\text{ liters}$)