How do I calculate new minimal speed for a glider when I put water into the tank?
$v_2 = v_1 \cdot \sqrt{m_2/m_1}$ is the correct formula.

$v_2 = v_1 \cdot m_2/m_1 = 87km/h$ is simply wrong, possible that it is only a typing mistake, but it is wrong just the same.