For these kinds of thing, consider the use of placebo. For those that swears something work, someone dump the miracle additive down the drain, and put in NSW instead. If after 6 months, and they still swear the NSF works like a charm, then you know it is not that useful.
For those that never use a particular "must-have" additive and still have nice tank, then it is more difficult to make a fool out of them. Maybe they are doing something they did not realize that is equivalent to adding additive, or there is something we all need to know about.