Ich is almost always in the tank as an unseen and dormant micro-critter. It can find it's host in smaller units of water (2 gallon vs a 20 gallon), and attacks when the host is stressed (stress lowers immunity).
Pre-dissolve AQ salt, and add. Yes, 1 tsp for your tank should do it, plus it will help with any possible ich. You'll want to siphon the bottom of the tank each day now to avoid any breeding ich at the bottom. Bring the water temperature to 80-85 if you can. (not only will your betta love 80 degrees but the ich will speed up the process, drop off, and the AQ will dehydrate and kill it!)