We threw about 8 grass carp in my parent's 1.5 acre pond back 15 years ago. They cleaned it up fast! Problem was, they basically ate everything green within reach and soon there was no vegetation left. After about three years, the grass carp were at least 10 lbs each. My father, being the problem solver he was, realized there were too many and got out the old military issue Mauser. You could never catch those things, they are very skittish. They do love to sit in the sun just below the surface. Well, he got all but one of them, which is all we needed in the first place. That one lasted at least 10 years and was 3-4 feet long at last sighting. Haven't seen him in 2-3 years though, so he must have finally died. Probably weighed over 50 lbs at peak, and yes, he did look like a submarine moving through the water. Could also put up a heck of a wave when startled, made for some funny reactions from fishermen that didn't know he was in there!