as far as i can tell, they're fine with it. i usually use 1tsp per 2.5gal if i do a 100% water change, or half that with a 50% water change, etc.
keep in mind, though, most betta medications are TOXIC to invertibrates. if you have to treat your fin for ANYthing, seclude the betta in a recovery tank. i found this out the hard way when some creep at petco gave me the all clear to medicate my entire tank, betta, shrimp and all. i had one betta, two ghosties and two bamboo shrimp, and only the betta survived.