Thanks, both. Yes, doing such an experiment with a high degree of accuracy would be time consuming as Raymond says, but doing it once in one light would at least be more informative than nothing. It's the kind of thing an aquarium club, for example, might find interesting as a project. Or a magazine for an article.
Burr740's experience of adding it to an existing tank and seeing growth take off is much closer, at least for the time just before and after you can presume all else stayed the same. On the other hand, the higher light tanks with only glut have a certain speculation that they would be an algae nightmare without. Maybe I should say informed speculation.
There are a number of reasons this is of interest. Once I get enough plant growth -- maybe I should stop dosing Excel, let things slow down. What happens when you quit -- do plants just slow down, or (like an addict going into withdrawal) do bad things happen. It's used for spot treating algae, but does it really, actually have an impact on algae at normal doses or is that mis-applied inference from spot treatment?
If you look at CO2 there's a ton of basic research out there about times and lighting and the interaction with water chemistry is very well understood. This "Liquid CO2" (which isn't of course) on the other hand is a very complex chemical with more modest impact, but is very widely used with not nearly the amount of publicized research to show its effect (and long term impact). Frankly speaking for myself, it could be doing absolutely nothing and I would have no way to know that. I suspect most of us using it just started and used it without ever doing it as a change from zero to compare. Maybe it's all placebo effect, like all the non-Celiac people not eating gluten and thinking it makes a difference and getting others to not eat gluten.
The more I searched the more surprised I was at that lack, and wondered if I was missing something.
I'm not stopping -- I still need a lot more growth before reaching "done" on my tanks, and certainly there is zero evidence out there that it does not work -- my comments above lament the lack of information, I am not trying to suggest it is nothing but flavored water!
But i wish I had time and space to set up a real test. Hey... any magazine editors listening -- think what a great article it would be. Get some author to do some testing!