Please help me with your knowledge of bettas and advice.
I have vieltail name Jhaybeebee and a crowntail name Sentou.
After taking them home and using the same water from the petstore for their new home (each in their own fish bowl of course), they seem to be happy - making bubbles, eating the given food, swimming around etc.
BUT...oh the big bad "but"

...but after the first change of water, both of them stopped eating. My vieltail, Jhaybeebee, just stays on the bottom of his bowl and barely moves. He'd swim to the surface once in a while but he seems to struggle and would just sink back down. Also, there's a fuzzy white thing over his eyes.

This was Jhaybeebee before the first water change. Notice his pretty fins nice and flared. My pretty Jhaybeebee!!

....but now his fins are always tucked in because he doesn't move much anymore

.
And something interesting(but still bad) is that it's the opposite for my crowntail, Sentou. He's now just floating! He struggles when he swims to the bottom and would just float back up! He doesn't eat and his color isn't vibrant anymore - he looks very pale now.

This was Sentou before the first water change. Notice his big tummy because he would eat A LOT!! lol

...but his big tummy is gone now since he doesn't eating anymore. Also notice his vibrant color in the picture? Now he almost looks completely gray

.
My guess is that the bowl conditioner that I used is maybe expired? Does it expire? It came with one of the fish bowl that I got for Christmas last year. I bought a new bottle of bowl conditioner today and changed their water right away.
Any suggestions and advices? Please help me.I'm super worried and I don't want to loose any bettas anymore. I honestly cried when my first betta pet died (don't laugh lol). RIP JJ - my red vieltail.

JJ - REST IN PEACE