Waco, welcome to the forum... Trying to remove enough bass from a 15 acre lake with an overcrowded bass population is going to be a real chore. I would recommend you keep a catch log. Measure all the lengths of fish caught and their weights. Then you can start calculating relative weights. I would keep ALL bass in the 8"-13" range. For a lake your size, I am estimating you're going to need to keep in excess of 750 bass that size for beginners. The fact that your sunfish population is still doing well, means you may just be getting to the crowded bass issue and not having been there for an extended period of time.