Yes, up-conversion into simple sugars is one possibility; with down-conversion to CO2 being the other.
Glutaraldehyde, according to Seachem, adsorbs into plant cells readily. Once inside the plant, anything it's converted to is also already inside the plant.
The ability of a substance to penetrate into living material isn't related to molecular size alone. As an example, consider dimethyl sulfoxide. It's molecular size isn't much different than any other common solvent; yet it penetrates so anomalously fast that you can put some anywhere on your skin, and less than a minute later taste it in your mouth (resembles garlic).
Afraid I can't provide any more detailed explanation than that. My chemistry knowledge is limited to what curiosity and necessity requires.
Thanks for your response. I agree that DMSO is a peculiar molecule. It has the properties of a high-penetrating agent (like ketone-based solvents) but acts as a carrier for other molecules through permeable barriers like cellular membranes. One of my undergrad degrees is in pre-professional biology, and I had a chemistry prof that did a series of lectures on DMSO, and even back then I though it was very peculiar stuff. I am actually going to look for my old notes from that class now that I have been reminded of it. But I digress.
I googled glutaraldehyde to see what I could learn about it, and that is a weird molecule too. Used for a bunch of stuff, apparently, including sterilizing medical equipment and water treatment among other things. Several articles actually noted its' use as a fertilizer for aquatic plants, but none of the articles actually had any hard scientific evidence that supported claims of its' viability in providing organically available carbon. All of the articles have something phrased like this: "It is claimed that..." and then the outline of the info provided in the Seachem blurb without actually having anything from a peer-reviewed journal to back it up. There was a ton of stuff though that discussed the hazards of it, most of it from either governmental regulatory agencies or reputable sources that discuss industrial pollutants. Apparently in higher concentrations it can be extremely toxic. Who knew?
I think I will continue to look into it a bit more. Thanks again for your response and the information you provided. Honestly, your explanation is a lot better than any other that I have read so far, so I am inclined to just go with that. My curiousity often gets the better of me, and a little knowledge can be a dangerous thing in my case, but it has been so long since those college chemistry classes that I think stuff like this is good because it keeps me sharp. Just don't ask me to do any stoichiometry. I guess the bottom line is that it works. I can definitely say that my plants are doing far better with it than without. That is the important thing, right?