Hi new here
I can see where the anode goes, the needle/pin, but where does the  cathode go please ( graphite block/pencil)

Hi, welcome to reef base.

It's here circled in red, just a bit of chrome plated metal I had lying around, probably off an old kitchen cup rack or something. It was just cobbled together with bits I could lay my hands on at the time. A cable tie and heat shrink sleeving is holding it to the tube.

There was a video of it in action but I had to delete it, on YouTube I got a torrent of abuse for animal cruelty...