This is a spin off of another post... I figured keeping the possibly controversial stuff out of someone else's thread would be the polite thing to do.
I keep seeing Excel being suggested as a first line of carbon supplementation and thought I would look a little closer at the product.
First off, I have not, nor do I plan on using any carbon supplementation or CO2 injection for my tanks. I run medium to heavily planted tanks with 12 to 14 hour photo periods with good growth and no nuisance algae. I use Flourish Comprehensive once every week or so and root tabs for swords. I don't believe that carbon supplementation is necessary in any but the higher energy systems where fast growth is desired for commercial or some other personal aesthetic reasons and then it is prudent to use a CO2 direct diffusion as it is almost twice as effective as Excel at carbon supplementation with zero byproducts (this right from Seachems).
The ingredient in the product that keeps getting negatively mentioned is Glutaraldehyde (chemical name 1,5-Pentanedial). To me, any aldehyde is suspect when being used for living organisms as they are used as preservatives, disinfectants and various microbiocidal applications. Here is an excerpt from the disposal section of Dow handling and storage guide for this chemical: "Glutaraldehyde-based solutions should not be discharged into lakes, streams, ponds, estuaries,
oceans, or other waters unless in accordance withlocal regulatory procedures [e.g.,U.S. National
Pollutant Discharge Elimination Systems (NPDES)permit]"
So, what about our nice closed aquarium systems? Adding this on a daily basis provides a regular source of a 2% aqueous solution of 1,5-Pentanedial (1,5-P) which might degrade over a 10 day period. A typical 5ml dose per 50 gallons results in an approximate 0.5ppm concentration of 1,5-P on day 1.
On day 30 the concentration will approach 5ppm but due to the stated bio-degradation rate of the 1,5-P, it will not get any higher.
The NOEC (No Observed Effect Concentration) is 5ppm based on 16 hour contact times with a variety of bacteria. Funny that it would work out to that, not coincidental I am sure. They don't test 24 hours a day for days, months or years. Inhibition of bacterial growth kicked in between 17 and 25ppm in the 16 hour trials.
I don't think that this will affect plants in any majorly detrimental way but I think that the fish might have something to say about us supplementing their breathing fluid with what amounts to embalming fluid... if they could talk.
If I chose to supplement carbon I would choose to use a CO2 injection system over adding Excel... or if I determined that I needed carbon to compensate for an off balance bio system I would choose to attempt re-balancing it first.