It appears Puriclean contains a 'chlorine based bleaching agent' according to the Material Safety Data Sheet - so it seems to be chlorine with added chemicals to make it into a powder or tablet. Why use this expensive method when you can buy bleach at 49p a large bottle? Off the shelf supermarket bleach is usually only about 5% active chlorine. So to disinfect your tank, just dilute the bleach with water in the tank so it is 50 parts per million (ppm) and leave for a few hours. It will kill off any bacteria. Make sure you flush out a few times with fresh water to remove residual chlorine and you should be good to go! With a 100 litre tank you will need about 65 ml of household bleach. Tanks do need cleaning - like any water system they grow bacteria.