You are going to lose some gold no matter what you do, the best way to try and save it is to construct an undercurrent classify the material so it is mostly fines and slow the water down, your classifier should do this. You could also jig your undercurrent a little this helps the heavier fines to drop out quicker.. it's very interesting to play with good miners Moss too really traps the fines..l the shape of the gold has a lot to do with recovery, thin flat pieces tend to float and sail over riffles it's the heavier looking pieces that are easier caught. . Then too no matter what you do your black sand production will increase but that's just part of gold recovery... Have fun I did. I learned a lot and was able to better recovery.