Thank you for your quick response! Yes, it has been drained several times in the past 20 years of using Baquacil. It was even drained 3' (leaving 1' of water) this year.

So, we will be replacing the filter sand tomorrow. Hopefully, that is the problem. It is too late in the season to drain the pool, so if need be, I will keep shocking it at night until I only lose 1ppm. What should I do during the day since even with the floating Trichlor, the FC goes down to zero?

Is my TA of 190 ok? My pH dropped down to 7.2 from 7.4...so am aerating again. It likes to stay at 7.0.

Next year, I would like to go with a SWC system. Any suggestions of brands? Also...do you have a favorite heater?

Thanks for your help.