Im not exactly what the problem is to be honest with you. If you are leaving with no less than 15 fish then I would say you're doing pretty well. I have started a couple ponds from the beginning to when someone else bought the land and I didnt take care of it anymore and if you have bass and crappie in the same small area, once you have dwindled the numbers of bigger size fish, the other predator fish (being bass) will end up taking over, maybe not completely but you wont see the same numbers you did before catching higher numbers of one species without catching the equal number of the other species. If the weeds are dying off in the winter and coming back in the summer then I would let it keep going the way its going because the fish are using it for cover. It honestly sounds like your pond is healthy and doing just fine if nobody is fishing it hardly and you have the good number of fish you say you do. The only thing I would watch is if the weeds start taking over and not going away and paying attention to whether or not the average size of fish gets smaller and smaller.