Tried with lead electrodes, 500 mL tap water and some NaOH to get about 15V, 0.7A, 11W.
No Carbon in the tank I take it? We have avoided strong bases btw, best results with carbonates - sodium or potassium - or even sodium bicarbonate, so not sure what might be happening in your tank. Sounds like pxidation/plating though. Tap water is fine btw.