Draining Tank Problem

Hi, i'm implementing a draining tank in simulink. The
original height starts from a memory block and is
decremented each iteration by a constant flow loss (which
is previously fed into an integrator and the inverse of the
area of the tank). The answer should be linear but i'm
recieving a nonlinear plot. also, i'm using ode45. does
anyone know where i'm going wrong?
.