I'm no expert on this device, but maybe if the metal pieces were flat (more closer, equidistant apart) instead, it might be more effective (more surface area of the "electricity field" since the electricity current would want to flow the path of least resistance (less water travel with flat discs rather than a ball, so electrolysis would happen across the whole metal mesh, rather than the outside edges of the ball that are closest to touching). I know this was just a quick DIY with spare parts you had on hand though.
Just a thought, no idea if it would make any difference or not (that resistance in water may not be a factor like I may think. I do hope water does have good enough resistance that the low voltage current is not "minorly" electrifying fish within the tank).