What do I use to stop small coolant leak in heater core?

To stop a small coolant leak in the heater core, you can use a commercial radiator sealant or coolant stop leak product, which is designed to seal small leaks by forming a barrier. Ensure the engine is cool before applying the product, and follow the manufacturer's instructions for best results. However, keep in mind that this is typically a temporary fix, and it's advisable to repair or replace the heater core for a permanent solution. Regularly check coolant levels and monitor for additional leaks.