Our salt reading went from 2800 down to around 2200 within a couple weeks. We added another bag of salt. It went back up to around 2700 and we added another bag of salt, bringing it up to 3000. Shortly after, we noticed the black algae. I read that the algae can affect chlorine production and I thought maybe our "salt" reading was actually measuring "chlorine" because the salt then declined within a couple days to 200. At that time I consulted the manual, which indicated I should inspect the cell and clean it if necessary. We were so clueless, though, we thought the plug that goes into the wall unit was the "cell", not the big white cannister. We pulled it out and dutifully checked it, and saw nothing wrong. Shortly afterwards the salt level suddenly dropped to 200. I then added 7 bags of salt (thinking the algae was still to blame) and within a day it dropped down to 0 and stayed at 0 for a week. Our control panel this entire time indicated "low salt", not "high salt". The cell is approximately 6 years old, which I think is a fairly normal life span?

Anyway, I just examined the old cell and it does look like it has some calcium buildup but I'm not sure what would be considered excessive. It's just a few crystals, not blocking the filter or anything. I didn't look at the inside of the new one before I installed it so I'm not sure what it's supposed to look like.

I do hope that I didn't just go out and spend $500 for a new cell that I didn't really need. But at least thanks to the helpful responses here I'm learning a lot more about my pool system and how to maintain it properly. I just wish it didn't have to be such a frustrating and expensive way to learn it. Thanks for your responses, and any additional insights would be appreciated.