gent you should go to a pool supply or a wood working place and get the peroxide there. it will be stronger, so you need less, but make sure to dilute it in some water before dumping it in.
if you use anything you will need to remove any screens in faucets as they will clog as the slime come outs. what i would do is shock it with bleach, a couple of cups, and run the faucets till you smell the bleach. let it sit over night, then drain completely. then repeat, the same then a couple of flushes to clear it out. once you do this open the inspection port if you have one, the bleach smell will go away in a day or so with the port open