The recipe states that you have to heat the fish until the geiger counter goes off, not until it starts glowing red.
ExplanationIn practice this would be absurd to achieve.
As you continue to heat an object, it starts to glow due to the infrared light it emits (the one we perceive as heat) that transitions to a red glow as the frequency emitted gets higher (that’s why that fish is glowing). If we continue to heat the fish until it reaches millions of degrees celcius, it’s frequency should shift to the ionizing range(radioactive), and set off the geiger counter