If your lighting, water quality, filtration or flow is less than optimal no amount of additives will provide much benefit. And this works both ways, if your system is the pinnacle of perfection then tampering with the ecological balance you've achieved may actually adversely effect the inhabitants of your tank. Basically its my belief that 99% of the 'additives' (apart from food and calcium carbonate supplements) available are a waste of money that should better be spent on aquarium equipment. That being said, I do use a little bit of Lugol's Solution each week.
I know, I'm a hypocrite.