if u use the wrong materials, u'll kill everything in your tank. if u apply the wrong voltage, u'll dissolve limestone in your tank (drop in PH)(chlorine is produce using this same method). those that really want to try electrolysis should just forget about it (but it can be done). unless u've got the correct anode if not, most materials that u use as an anode will just dissolve right in front of your eyes.
6mm rebars will most likely last for only a few days and your tank will be completely brown due to Fe that has dissolved.
u'll need the most presize charger, + - . ? amp different makes a big difference.
exotic compound coated on titanium will only last 5 or more yrs as anode before it all dissolves into the water. anyway most ppl will never be able to get this material. (it's patented.... there is 2 companys in the world that produce them now) price is about 100-150 USD/m2 for retailers excluding frieght charges.
cheers