Drain and refill here, in balmy MN today! I have a few light scratches in my shell due to the plastic on some seat boosters and whatever else the kids drag in....
You can see them when "stuff" collects in them, and even shocking didn't bleach them out. I have heard you all talk about using the Magic Eraser, so I finally tried it today. I used the generic Target version. It removed the slight stain at the water line and took the "stuff" out of the scratches!!
The birds and squirrels are enjoying the warm draining water, and this will get me through until March--wahoo!!