I have taken care of my pool according to the tfp method for years now and noticed more or less this year rust around the screw holes that are under water. The screws aren't rusted after pulling a couple out but just some bleed over rust stain. the threads are clean. I keep ph at 7.3-7.7 for the most part and alkalinity between 70-80. I use liquid shock to chlorinate. I don't run water through the heater until it is well balanced in the spring. I am thinking it is water chemistry but can't figure out what it could be except copper in the water? Note: I do have an iron issue although not terribly bad I will deal with soon using AA and a sequestering agent.