We study the total variation (TV) distance between the laws of the 2D Ising/FK-Ising model in a box of side-length $N$ with and without an i.i.d.\ Gaussian external field with variance $ε^2$. Letting the external field strength $ε= ε(N)$ depend on the size of the box, we derive a phase transition for each model depending on the order of $ε(N)$. For the random field Ising model, the critical order for $ε$ is $N^{-1}$. For the random field FK-Ising model, the critical order depends on the temperature regime: for $T>T_c$, $T=T_c$ and $T\in (0, T_c)$ the critical order for $ε$ is, respectively, $N^{-\frac{1}{2}}$, $N^{-\frac{15}{16}}$ and $N^{-1}$. In each case, as $N \to \infty$ the TV distance under consideration converges to $1$ when $ε$ is above the respective critical order and converges to $0$ when below.