That mix is too hot! You want Miracle Grow Organic Choice potting Mix.
Please take the Betta out asap to avoid finrot and possible death.
I'd just dump that soil out. It isn't going to work for this. I've done alot of walstad tanks and small vases/bowls.
You really need to go with organic potting mix to avoid these issues.
Also, for a proper walstad bowl, you need tons more plant mass. Fast growing stems, rosettes, and alot of floaters.