Remove the beta fish using net and put in in another small water container, remove the water pour the clean water and put the fish again into the tank.
The first place I would look is in my owners manual. If thats not available and you know the companies name try contacting them either by phone or online
There are many products on the market that offer complete water conditioner. Be sure and choose a kit with the correct amount of cleaners for the volume of your tank so as not to add too many chemicals for your tank.
Clean the injector
Turn the water softener control to the bypass mode. Unscrew the softener injector cap and remove the screen and injector nozzle. Remove and clean the screen with warm soapy water, then rinse in clear water. If it is damaged, replace the screen. Clean the nozzle with canned air, a small wire, or a paper clip.
tap the water inside the tank first. then remove the harmful chlorine heavy metals and chloramine. make a helthy slime coat to protect the fish from the effect of ammonia.