Betta Fish: why did she die?

so she was just getting her color. she was fine up until last night/ this morning. i was told to wait it out and watch her. found her sitting up dead at the bottom of the tank. so why? i was treating the tank with paraguard for 7 days before taking the one platy out and stopped treating, as i ran out and nobody else was showing signs/ i was treating for new fish. i did a 20-30% water change yesterday. ammonia reads 0 nitrites reads 0 nitrates reads 0-5ppm. you can see there is tannins in the water so idk if that effects reading. the bubbler was cut to a low flow due to her needing to swim. i feel the flow was still to high but it wasn’t bad. she lived with platys and mystery snails. i have a female in my 20gal long and a female in a temporary cycled 5gal. do i even bother putting the female that’s in the 5gal in the 55? or will she suffer the same fate. fish keeping is so unrewarding. i’m about to throw my tanks in the trash i’ve lost so much money.