Doctor Duck, if I were you, I would not add any additional GC. I overstocked my pond with them (180 in 6 acres of water) and as a result they have denuded the pond of vegetation. The nutrients which would otherwise go into healthy vegetation are instead taken up by phytoplankton. That means that each summer through fall the lake turns an ugly, murky, brownish-green. What was once inviting now looks icky.
The youngest of the GC is 23 years old! They seem to never die. Last week the water was exceptionally clear (it won't last) and I was able to count 16 of them in one small area. I've tried shooting them, I've tried catching them with every vegetable imaginable (cherry tomatoes included), and I've even put a bounty of $25 each on their heads for the neighbors to try their luck. As has been noted above though, they are extremely elusive. In the past 5 years we've only been able to confirm 3 killed. We may have killed a couple of more but they tend to sink fast when shot.
Do not believe the hypothesis that when they get old they stop eating! I only wish it were true! Here is a photo shot last week showing a fence we constructed to protect a tiny cove in the hope of reestablishing plants. The GC cannot get into the area on the left. The area on the right where the GC lurk looks OK now, but it's only May. Soon the water will turn ugly.
