Sean, I don't think the heater is the problem because we haven't used it since we put in the SWG. Also, the SWG is plumbed after the heater. This morning I put vitamin c on the return jet. It removed the yellowing instantly. I suspect that I have metals in my pool because I use well water to fill. I put some metal out in the pool this morning. I hope that takes care of the problem. The forum dealing with stains gave me this idea. Thanks, Dorie