Welcome to SHOP Specialized and focused on aquarium products! If the temperature you set is ten degrees warmer than room temperature, choose a larger size Example: I have the 15L aquarium, I buy the 25W heater. But, it’s cold, the air temperature is 8 degrees, I want to heat the water up to 20 degrees, I have to buy 50W which is more power because the air temperature is more 10 degrees than the water temperature < 20L of water choose 25W < 50L of water choose 50W
