Used as recommended, there's not much harm it will do. However, we often use two and three times the recommended dose, which can definitely be harmful over time.
Furthermore, there are two different doses - startup and maintenance. The startup dose is fairly deadly if used regularly, and some people mistakenly do so.
In addition, the directions on the bottle (and I'm trying to quote this from memory) say to use it "daily or every other day". Which, clearly, is extremely precise, which doesn't help.
Finally, certain fauna - especially inverts - and some flora (the generally fragile types) don't respond well to Excel, though, again, the basic daily dose shouldn't do any harm.
*edit* One other thing - and this is mostly speculation on my part - the length of time that glutaraldehyde (the active ingredient in Excel) remains potent in the water likely varies. I've read anywhere from 12 to 48 hours, and that might depend on total amount added, lighting duration and intensity (it's photosensitive), and carbon uptake of the plants ...which, of course, will decrease in the event of an Excel overdose. So, if it's 12 or 24 hours, no problem. But if it's 48 hours, then you're dramatically increasing the total amount of glut in the water column if you dose daily. Just food for thought.
P.S. the most common use of glutaraldehyde is as a sanitizer (google Cidex and Metricide). Meaning, we use it to kill stuff. Remember that as you add it to your ecosystem.
Last edited by kevmo911; 07-04-2012 at 11:07 PM.
Reason: added info