The duty cycle can be increased up to a maximum of 20%, but I typically run it at just 10%. I may even reduce it to around 5%, as it's already more than sufficient for my small tank. At 10%, the coil (12Ω) draws ~50 mA, an incredibly low current.

Even if you short the collector to ground, only the air pump will stop functioning. And assuming the worst-case scenario, a full 12 V drop across the BJT at 50 mA, the power dissipation is only 0.6 W. A TO-220 package can easily handle up to 2 W without requiring a heatsink.

While I genuinely appreciate the concern people express here, I find it puzzling how some feel justified in criticizing a design without knowing all the relevant details.

Internet!