I would add that it's not just the automated puck feeders that can be a problem (if not used properly), but also the standard "floating dispensers". Long before joining this forum, I stupidly used Tri-Chlor tablets as my sole chlorine source and had them in a floating dispenser. Since I had an automatic opaque pool cover and did not want the floater to always end up at the shallow end of the pool, I tied it to a length of string attached to one of two bars we have in the pool along the side just below the water line. I usually closed the cover and then manually pushed (from the top of the cover where I could "see" the dispenser's outline) the dispenser more into the middle of the pool, but sometimes I forgot or just got lazy and let the cover sort of push the dispenser to the side of the pool next to the bars.

Well, needless to say, I ended up with rust in the two anchor posts closest to where the dispenser parked itself. My pool pump ran 8-10 hours/day so I'm sure that the bulk of the time when the water was "idle" and not circulating, acid built up from the Tri-Chlor tablets and corroded the metal that was nearby.

So even a seemingly innocent floating dispenser can be a menace in the pool due to the very acidic nature of Tri-Chlor tablets.

Richard