Cooling System Additive

The Dart heats up a little when in stop and go when it's 90-95 degrees out so I try some water wetter a few years back and it did make a difference. I changed the coolant lately and decided to try Purple Ice. Didn't read the directions before buying it. Took 1.5 bottles verses one bottle Water Wetter and the directions say you have to add it again every year or 30,000 miles. Water Wetter didn't say that. I asked Purple why and am waiting for them to get back to me. I wanted to know what is wearing out. Just the rust inhibitors and/or the surfactant.