If the tanks are at least 2-2.5 gallons each, they will likely do much better then something smaller.
Also, it's good to know the tank size so you know how often to change the water. Clean water is the key to a happy betta! If it's 1/2-1 gallon you should change the water every few days. If you have a 2-2.5 or larger, one to two times a week would be fine.
One reason the goldfish died could have been the size of the tank. Goldfish usually needs about 20+ gallon tanks to thrive.
Also, I'd think that using water conditioner in tap would would be easier and more cost efficient. I prefer Prime.
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