Betta Died - Causw?

2nd betta dead in 3 months.

First one was likely my fault due to over feeding blood worms.

But second one seemingly just died?

Day before he was active and excited to eat, then yesterday seemingly lost control, couldn’t stay right side up or swim about. Then sunk to the bottom and just died.

Water conditions are fine except for PH which is about 9 - would this cause him to just change and die within 24 hours of being apparently healthy?