OK, sorry. Read too quickly (that's the trouble with putting the subject *only* in the heading)

What water butt treatment are you thinking of? ie what are you trying to treat it for? I've googled and found "enzyme technology to prevent unsightly scums, eliminate unpleasant odours and helps keep the water slime and algae free. It is also specially formulated to control soaps and body fats."

Scum, slime and odour is something you shouldn't have if you're collecting rainwater, algae is not a problem if light doesn't get in. So are you looking for a treatment because you're planning to collect used bathwater?