Having recently stumbled across various discussion threads relating to the use of Glutaraldehyde in the aquarium to promote aquatic plant growth, I thought I would research the topic a little more for my own edification. This is what I came up with. The commercially available product by Seachem called Flourish Excel TM has a rather oddly termed compound called ‘Polycycloglutaracetal’. Based on the curiously formulated name, it appears that Seachem just made it up to describe a concoction of aqueous Glutaraldehyde with one or more chemicals of undisclosed nature. It therefore follows, that you will not find this fictitious compound on any MSDS or official chemical register of compounds. Of course it is widely known that glutaraldehyde has algicidal properties at specific concentrations, along with uses as a fixative for electron microscopy. Glutaraldehyde is a small compound made up of a short carbon chain with an aldehyde functional group at each end. The chemical formula is HCO-(CH2)3-CHO. The terminal aldehyde groups are quite reactive and in aqueous (i.e. in water) form (> pH 7.0) glutaraldehyde molecules readily forms cross-links to form polymers of varying length. These oligo-/polymers also readily combine with nitrogen groups in proteins to form additional cross-links. Hence, this is the likely origin of ‘Poly-‘ and ‘–cyclo-‘ in the name ‘Polycycloglutaracetal’; the rest of the name is self-evident. So, as it would appear, Seachem has formulated the name to describe the behaviour of glutaraldehyde in water with the additional of some type of protein or other organic compound. The polymerisation capacity of glutaraldehyde to proteins is widely used in biomedical fields in regeneration of collagen and ligaments. Moreover, cross-linking of aqueous Glutaraldehyde with proteins involves more than a dozen different forms (e.g. isomers) depending on solution conditions (e.g. pH, temperature, etc.). These isomers are in equilibrium, so whatever isomer predominates in solution will depend on ambient conditions, and appears not to influence the beneficial net effect of the compound to plant growth. The figure provided by Seachem to describe the general structure of ‘Polycycloglutaracetal’ also corresponds well to the rationale proposed here (see [url=http://www.seachem.com/support/Articles/CO2PlantedAquarium.html).
The bottom line is that yes, there is a positive effect on plant growth by judicious addition of Glutaraldehyde to aquariums as an apparent supplemental carbon source, especially in combination with gaseous C02. For proprietary reasons, Seachem has been understandably coy about disclosing the actual formulation, but it appears that the actual chemistry is not as mystical as it would seem. ‘Polycycloglutaracetal’ is likely a rather stable, and safe, formulation of aqueous glutaraldehyde with one or more undisclosed organic compounds that will undoubtedly continue to have a measurable benefit of promoting vigorous plant growth in aquariums. I hope this helps dispel some prevailing misconceptions and I would be very interested if anyone has found out anything to the contrary or in addition to my conclusions about glutaraldehyde and Seachem’s products for promoting plant growth in aquaria.

SLW [plants first,... fish will follow]