As a dislaimer, I use this product. The “BeAware” reference is not to discourage it's purchase or use. It is to bring attention to hazards associated with this product if used incorrectly.
SeaChem Excel is a very functional product sold as a CO2 substitute, or supplement, for planted aquariums. My experiences with this product are that it does, as advertised, provide benefit, when used according to directions in planted aquariums. Especially in low light planted aquariums. However, like so many products sold within the aquarium hobby, a review of active ingredients is a little concerning.
Listed as the “Active Ingredient” on the bottle is Polycycloglutaracetal. Research reveals that, aside from SeaChem literature, there is no such thing. Polycycloglutaracetal, apparently, is a trade name for a product developed by SeaChem, which appears to be an isomeric form of glutaraldehyde. At least the Material Safety Data Sheet (MSDS) for Excel list glutaraldehyde as the active ingredient.
Glutaraldehyde is an antimicrobial, bactericide, fungicide, and virucide, commonly used to sterilize medical instruments. It is also used as an embalming fluid, as an ingredient in Anti-Freeze, an antibacterial agent in cooling towers, a leather tanning agent, a biocide in water treatment, a sanitary solution for portable toilets, and is used to sterilize ballast tanks in ships moving from one water source to another (to kill off pathogens and critters that may be transferred in the tanks from one water way to another).
Glutaraldehyde is also an algaecide. Amongst planted tank enthusiasts, Excel is commonly used to defeat undesirable algae growth. I have personally witnessed a massive decline of algae following use of SeaChem Excel. Originally I attributed this decline of algae to Excel stimulating plant growth resulting in the plants out competing algae for nutrients, starving it. After further research I am not so sure. There are too many reports of green water tanks, without plants, being cleared by use of SeaChem Excel. Combine this with the fact that glutaraldehyde is a known algaecide and duh!
There are also reports (and something else I have witnessed myself) that higher levels of nitrates are experienced after use of the product. While this topic (higher nitrates) remains under discussion, I am of the opinion that use of SeaChem Excel may result in higher nitrates for two reasons. First off, it obviously does kill algae and the decaying algae will contribute to higher nitrates. Secondly, I am of the opinion that some measure of denitrification occurs in most tanks with a substrate in excess of 2” and/or containing the appropriate types of filter media within canister filters. Being a sterilizing agent, it is safe to assume that glutaraldehyde would have a negative effect on the limited colonies of denitrifying bacteria within our tanks. This effect on denitrifying bacteria may occur if used according to dosing instructions. I am not aware of any affect on nitrifying bacteria but it is a given that overdosing will affect these bacteria as well.
I began this article with a statement that SeaChem Excel is a functional product. It is. It does do what it claims to do, providing an alternative form of organic carbon for plants in an aquarium (the actual chemistry of which no-one, including SeaChem, knows). When used according to the instructions the product should be perfectly safe, although there are reports of some plants that do not do well, or even die off, when dosed with Excel (Anacharis and Vals). There are also reports of invertebrate die off following its use.
However, this is only part of the story.
The active ingredients within Excel are potentially very toxic and care must be taken in its use. Glutaraldehyde will chemically burn skin, can damage your lungs if inhaled, and, if you somehow manage to get it in your eyes, can cause serious damage. This product contains a childproof cap for a reason. If used excessively (overdosed), it can, will, and has killed fish and plants. The maximum dosage would be twice the recommendation. However, I would not be comfortable even with that dosage. At the most, I would dose at the recommended levels, using the lesser of the two options (1 capfull per 50 gallons, every other day). It should be noted that at this dosing level, it will not function as an algacide. It is only functional in killing algae if used in excess (2 capfulls per 50 gallons, per day).
In closing, I would like to clarify that SeaChem is a reputable company in which I place tremendous trust. They are one of the few “vendors of choice” I use. Under no circumstances am I attempting to negatively review Excel as a product. I simply want the “Buyer to BeAware” be aware of the potential hazards use of this product present.
Always research a product before you buy it and use it.