I don't really want to be arguing the minutia of what the law says or does not say, I will leave that to the lawyers.
However that is not the issue. the OP (and I attach the quotes below) was talking of actively experimenting where there was the potential to cause harm ( which is the issue) not whether it will or wont or does or does not cause harm. It the potential for harm which is the issue.
Take
@Colin_T 's example of filling from the tap one day and the fish go belly up
As I said before, its ok do trial different feed additives in different capacities or change your lighting or water change regime, but when It has the potential to cause harm then there is an issue.