I'm guessing you have several miles on the car. ON my 97 civic EX, my side tanks on my radiator started leaking, and was replaced. Also, on a 94 Accord, it was losing coolant but I couldn't find where. So someone recommended using powder aluminum in the coolant which is intended to stop leaks. Ended up SHOWING me where the leak was as it make the leak worse: the top tank of the radiator. It was too far gone for a simple fix like that so a new radiator was installed and fixed the prob.