The short answer is "No, Flourish Excel™ is a beneficial carbon supplement that is typically harmless to tank inhabitants and plants alike." We'll address some of the specific concerns and claims that we sometimes find on fishkeeping forums below:
Claim 1: Flourish Excel™ cannot be used with CO2, and if you use Flourish Excel™ instead of CO2, your plants will die!
We in no way claim that using Flourish Excel™ is a “replacement” for using a CO2 system or even for CO2 that is accounted for by simple atmospheric exchange. We also do not make any implication that you cannot or should not use CO2 systems when using Flourish Excel™, in fact, often times using the two in conjunction is great for heavily planted tanks where dosing more from a CO2 system would potentially cause a harmful drop in pH and KH. Seachem makes no claims and certainly doesn’t advise against the use of Flourish Excel™ with CO2 systems by any means.
Flourish Excel™ is an excellent carbon supplement, but we freely admit that the growth results you get from using Flourish Excel™ are not as dramatic as the results you can get from a CO2 injection system. If a full-scale CO2 injection system with pH sensor, timer, and diffuser is a 10, then Flourish Excel™ is a 7. Flourish Excel™ provides a source of carbon, just as CO2 does, and carbon is essential for plant growth. When used as recommended (no gross overdoses, etc) Flourish Excel™ is safe for fish, plants, and invertebrates, and provides a beneficial source of carbon to the plants.
Claim 2: Flourish Excel™ is not actually Liquid CO2 or Liquid Carbon!
“Liquid carbon” or "liquid CO2" are misnomers that have been applied to Flourish Excel™ and similar products by our competitors and the general public to quickly summarize the purpose of the product. We don't use those terms in our packaging or marketing because they are inaccurate. Flourish Excel™, which is the product’s name, is an organic carbon source, that yes is in a liquid form. We in no way claim that is is a straight liquid form of elemental carbon (which does not exist) or CO2 (which only exists at high pressure or low temperature, such as in a CO2 canister).
Claim 3: Flourish Excel™ is actually gluteraldehyde! It is a dangerous carcinogen that is harmful to fish, plants, and humans!
This claim, that Flourish Excel™ is Gluteraldehyde, is a common one that is simply not true. I cannot speak for any other companies that may or may not use Gluteraldehyde, I can only speak for our company and our product. While structurally similar, Flourish Excel™ is NOT a gluteraldehyde product. Flourish Excel™ contains a molecule (2.0% polycycloglutaracetal) which contains a 5-carbon chain backbone. This molecule simulates the way a carbon would function in photosynthesis and helps to fill in the gap in a plant’s metabolic chain when there is not enough carbon present in the system (whether using a CO2 system or just from atmospheric exchange). Again, this isn’t to suggest that Flourish Excel™ replaces a CO2 system or atmospheric exchange or will interfere with either of those methods. It’s is simply an addition to the Carbon supplied by those methods, should extra be required.
Here is the SDS sheet for Flourish Excel™. Polycycloglutaracetal is not a carcinogen.
Claim 4: Flourish Excel™ is actually just an algaecide and does not actually help the plants grow!
Flourish Excel™ is not an algicide, we do not recommend it as an algaecide, and make no claim for it being an algaecide - because it’s not. Carbon is a fundamental limiting factor in an aquarium environment, where carbon is only available through gas exchange at the surface, unless you use some other means of carbon introduction. In these such systems, carbon is often the number 1 limiting factor surpassing plant growth. Because of this deficiency, the metabolism and photosynthesis of the plant suffers, causing a back up of other nutrients. Algae, are very good at taking advantage of this and blooming when there is a deficiency of some major nutrient (such as carbon). So, when you add a supplement of carbon, whether this be Flourish Excel™ or a CO2 system, you fill that metabolic gap, the plants can function and process nutrients again, and algae tends to die out because the plants are now functioning properly.