Houston, we have a problem. I am losing chlorine overnight. Please advise on next steps.

I presume the following. Drain the pool as much as possible. Open up the filter and throughly clean the grids.

Refill and chlorinate/stabilize

Experts please help.

I have owned this pool for 10 years. The first 8 years were a breeze. The last 2 have been difficult. The issue this year has never happened before.

Thanks!