"Liquid CO2 Myth" First there IS no “liquid CO2" at normal air pressure. If it’s in a bottle on your shelf, it is NOT carbon dioxide. Carbon dioxide is ONLY liquid if it is under high pressure, as in a CO2 tank from a gas supply company.
Next, there is the recent practice of using generic gultaraldehyde rather than Excel or other commercial products. This is risky business. Read OSHA's safety requirements and ask yourself whether aquarists are handling it this carefully: https://www.osha.gov/Publications/glutaraldehyde.pdf Then, if you can wade through it, read this study on exposure in rats and mice. Not pretty: http://ntp.niehs.nih.gov/ntp/htdocs/lt_rpts/tr490.pdf That's why I am adamantly against people using generic Gutaraldehyde. The risks are just too high. This is a known neurotoxin, and every exposure makes adverse effects more likely.
Now to Excel, which is MUCH less concentrated, and I believe has a slightly different chemical make-up. As far as I'm concerned as well as some very knowledgeable PhD level scientists, it is actually a mild algaecide, which kills some or all of the microscopic film of algae that grows on plants kept under less than ideal conditions. (if you don't believe this happens, look at a leaf under a microscope sometime) This in turn allows the plant ,itself, better access to the nutrients and CO2 that ARE available within the system. So the plants do benefit from Excel under some circumstances, but not for the reasons stated on the bottle. I do not believe that plants use Excel (or Glutaraldehyde) directly as a "carbon source". If anyone has a SCIENTIFIC STUDY that proves that it does, I'd love to see it. BUT, if you can't or won’t provide a source of supplemental CO2 in your tank, Excel (or similar commercial products) can help you get better growth. But please stick to commercially available products, use in a well-ventilated area, do not get the bottle near your face, and do not get it on your skin or in your eyes. If you do, make sure to flush immediately and thoroughly in running water. If you develop a rash or respiratory symptoms, contact your doctor immediately.
Seachem is a good company, and they deserve a lot of respect. They were the FIRST company to really listen to planted tank aquarists and give us the products we wanted. They are "the good guys" in my book. I don't agree with them on how Excel works, (based on the work of other scientists at the University of Copenhagen) but as I said, it does improve plant growth in certain circumstances. I was a beta tester for the product, so I know as well as anyone how it works. I have problems with how people use it, (at larger than manufacturer recommended doses, which they never suggested) and problems with misinformed people who insist on calling it "Liquid CO2". (again, something Seachem has never suggested) My biggest problem, and it is due to serious, documented health concerns, is with people who buy and use generic glutaraldehyde just because it's less expensive, and suggest that others do the same, without significant warnings on how to handle a dangerous chemical safely.
After I wrote the above on Face Book, Gary Lange wrote this response:
"I guess I missed this thread the first time around and saw it in Cheryl's "March Clippings". (a member’s only newsletter for the Aquatic Gardeners Association) It's one of my pet peeves, the ignorance of using glutaraldehyde and then thinking it's the same thing as Excel. Excel is a crossed linked group of glutaraldehyde molecules so it is not near so reactive as unlinked material, it only has a few "active" sites. However there most likely are still uncrossed-linked molecules in the solution. The fact that some types of fish die from it suggests that. Saying that glutaraldehyde is just like Excel is really like saying a solution of 20 of the known amino acids is just like insulin. Insulin is a cross-linked protein chain consisting of amino acids and no way will you get the same effect with a bunch of amino acids in solution. Whoever made the leap from Excel being equal to glutaraldehyde in the first place needs a really hardy "Gibbs slap" (for all of the NCIS fans) on the back of the head! In the biochemistry lab we use glutaraldehyde to cross-link small molecules that we can't make antibodies to with larger molecules. A reactive glutaraldehyde molecule will do this. When our solution gets too old or some goober in the lab doesn't tighten the lid it eventually reacts with air and then cross-links, it's a slow process in that manner. When you inhale small amounts of glutaraldehyde it cross links your lung tissue. We also use formaldehyde for these sorts of fixing processes and everyone is pretty aware of how dangerous formaldehyde is or they should be. Now the other problem is when you get active material on your skin it can be absorbed and start cross-linking some of your proteins in your skin. Probably not good but most of the time your body just does the repair work and moves on. But you COULD cause a problem where your body no longer recognizes this cross-linked protein and now decides it's a foreign host and this could lead to an autoimmune response. If you dig Lupus, go for it. As someone who worked with it for years in the lab and handled it VERY cautiously (in a fume hood) I think there isn't anything dumber than using glutaraldehyde in your aquarium. I think you would be better using a broken heater instead, at least you would get the instant feedback!"