Personally, I wouldn't use the Bettafix medication-
Tannins-IAL (Indian almond leaf) usually you have to get this online.
I use naturally dried and fallen from the tree Oak leaf-about 20 crushed per gallon of water. The leaves need to steep for at least 30min to start the release of the tannins. The longer they steep- the more tannin released- the darker the water -the better for the Betta.
The tannins have antibacterial/fungal properties and can help ease stress.
You can adjust the water at the tap for the correct temp-be sure and use an independent thermometer to check the water temp. The extreme, sudden temp swings can be stressful if not deadly on a compromised Betta.
Be sure and use double dose on the dechlorinator.
If you bought her in one of those small temporary cups with a lid-these work great for a QT-but anything small, shallow will work that can be floated (attached) in the heated tank.