It's really tough to legally launch a commercial product in the United States. You have to comply with all of the federal and various state's licensing requirements.
Even SeaChem briefly quit selling in one of the states (Wisconsin I believe it was) because the state regulators there were not satisfied with their labeling requirements.
Companies like Tropica, Seachem, and Plant Products ... have all filed all of the federal and state regulatory paperwork which includes permits, fees, and testing ... a relatively expensive and time consuming process...
It is something I explored repeatedly and could never find a viable economic model to justify pursuing it ...
I always thought it was a great idea ... but could never find a viable economic model that could cover the compliance costs. And for years I had a basement full of empty dosing bottles to prove it <grin>...
Now I don't know whether he his still doing it or not ... but I was fascinated when he tested an idea that I had previously tried and was unsuccessful with ... where he placed the do-it-yourself dry ingredients in a plastic bottle and shipped you the bottle with instructions to just add water ... I was sort of surprised when that idea "flopped" for me - but I can sort of understand why, because the cost of the bottle is about 100 times the cost of the individual plant nutrients that go into it ... I still think the convenience of having a nice dosing bottle should have made that idea work, not to mention that it saves the shipping cost of not having to ship water when compared to other commercial liquid alternatives...
If someone wants to send me a set of his products, I would be happy to test them out and report back ... I have tested just about every commercial product I am aware of - and each seem to have their positive characteristics. When I test, I test against both hard water and softwater, high light and low light, and a variety of different plant species. You will OFTEN get very different results just from those few variations.
A DIY approach that I normally use gives you a cost advantage because it is cheaper - HOWEVER - if cost isn't an issue, then convenience becomes an extremely valuable factor - everything else being equal of course.
The key is to find the right product that works well for you with your unique environmental variables and lifestyle!
Greg Watson