Ammonia, nitrite and nitrate levels. Do you have a test kit yet? I do know that mollies have a pretty large bioload, so you should watch your levels closely and do water changes accordingly. Also, it is recommended to have 2 or 3 females per male when you a keeping livebearers to keep the males from pestering the females to death over mating. However, in a 17 gallon, I wouldn't recommend adding any more mollies, if you notice the females getting stressed perhaps you should return a male for another female. Also beware of over feeding, as uneaten food will decompose and produce more ammonia.