Betta Fish: Nefertari died, entirely lost as to the reason. Would love some opinions.

Over the course of about 10 days, Nefertari became sluggish, timid, and lacking any appetite even when food was touched to her mouth. She showed no signs of bloat, dropsy or otherwise. Her color had faded slightly but other than that she showed no external signs of illness. She wasnt seeming to lose weight and was swimming to the surface and resting spots, albeit slowly.

I did a water change after the second day of her issues because that was when she outright refused food rather than spat it out. I did it with R/O to address the one issue I noticed with her water(hardness was high but it had been high since day one) but otherwise her parameters were 0-0-0 ph 7.2.

Her 10 gallon filled to 8.5g tank was at 78° through the course of her illness, tested with an instant read thermometer. inhabited by her, snails, an amano shrimp, 4 ember tetras. There was a large piece of mopani wood with anubias thriving on it, and water lettuce she liked to nap in. Catappa, oak, and magnolia leaves and crape myrtle bark and (organic) rooibos tea were used to keep her water nice n dark.

The ember tetras and amano shrimp are still showing no signs of stress so I dont think there is some kind of illness in the tank.or weird parameter imbalance cause Im struggling to put the word "tough" next to ember tetras.