Muddy water blocks light, weeds don't grow. That's probably too many grass carp for a small pond. I would limit it to no more than two, maybe get them out entirely. When a pond fluctuates that much, you will always have bare earth on the banks when it's low.
Maybe you could find some discarded astroturf or similar, and line the banks with that up to the full mark, or have two inch crushed gravel hauled in to line.
I have one 1/4 acre pond that fluctuates about 30 inches due to a slow leak, and have the same problem with bare banks in summer, but not muddy water. The banks are hard clay, so not much erosion there, but it does stick to feet. There are some pond weeds, but it won't get thick enough to line the banks above the water line. Small ponds don't really have wave action that erodes banks like a large pond or lake does.
