Use a thin acrylic rod, feeding sticks work well, and prod around the eels hole. It will eventually leave the sandbed, they are poor swimmers and very easy to catch in open water. I caught 9 this way during a tank transfer. It took a few minutes to get the method down but once I figured how to prod correctly, I removed all eels in under 10 minutes.
I may have misinterpreted your post, disregard if so, but you can not expose Garden eels to copper- it will kill them.