You have to take into account the total volume of water you are trying to heat, not just the water in your sump. It's constantly circulating, so where you apply the heat will not make a huge difference. If you had a 20 gallon display tank with a 180G sump, you would still need the same wattage to heat the water as you would in your setup with a 180G display tank and a 20 gallon sump. You are heating 200 Gallons any way you look at it. You are a bit below the standard 2 to 4 watts per gallon rule of thumb, but if it's been working for you and you see that your heater is not always on, I don't see why you would need to add another heater. Having said that, it is always good to have a spare heater on hand just in case, or more realistically, when it inevitably stops working. Make a habit of checking your temps because heaters are one of the more faulty products in the hobby.