Unfortunately the way you have it setup, even with a float switch and a strong house pump, you'd still likely empty the tank and kill the fish. I actually had that happen once, but luckily I checked the pond every morning. Woke up one morning to find my pond only had about 3 inches of water left in it, the fish almost dead, my 1000l water tank empty, and my house pump was shut down by the thermostatic switch.
I then changed everything around so that the pump sat in the pond and sprayed water vertically and back down into the pond.