Mom inherited late husband's koi

Hello friends, My elderly mother's husband recently passed away which left her in charge of his raised koi pond in the back yard. It was his passion project, and it means so much to her to care for it. It's very nice with a plants and a small waterfall.

My uncle taught her how to backwash the pump, add algecide and change the filter. Her husband only used to do the backwash once a month, but my uncle told her to do it weekly. Suddenly, now---after about a month ---the fish are beginning to die.

The PH was off, so she's slowly adding baking soda to the pond which is evening leveling it out. She told me the fish weren't eating and staying close to the bottom so she knew the PH was funny already.

Is this because she is backwashing more frequently than her husband had/too frequently? Is it the algecide? Any ideas? My mother is 80 and it's breaking her heart watching her husband's fish die off so recently after losing him. I'll take any advice, Thank you so much!

(For the record I live overseas from her, but am trying to get information to help as much as I can)