If V(t) is the volume at time t, \(\displaystyle \alpha(V)\) is the rate, depending upon the volume, at which the tank drains, and \(\displaystyle beta\) is the rate at which the tank is being filled, then
\(\displaystyle \frac{dV}{dt}= \beta- \alpha(V)\).
Without knowing more about how the rate at which the tank drains "depends on the volume", we cannot say more. If the rate at which the tank drains is proportional to the volume, then \(\displaystyle \alpha(V)= k V\) for constant k and the equation is relatively easy to solve.