I put 10 grass carp into my 1.5 acre pond 3-4 years ago to control algae etc. They are now at least all 15-20 lbs and I have always read that they are most effective the first couple of years - thoughts on how to get rid of these huge guys and then add some new youngsters? I tried shooting them with a 22 when they are visible but no luck - the water obviously reflects the bullets - suggestions??