Liquid carbon products: what the labels claim and what is established

What “liquid carbon” is sold as

Plants build their tissue from carbon, which they normally take from dissolved CO2; Tropica calls CO2 the most important nutrient, without which plants cannot photosynthesize ^[5]. “Liquid carbon” products are bottled alternatives sold for tanks without CO2 injection, or alongside it.

Seachem’s description of Flourish Excel is that it is “a source of bioavailable organic carbon”, a simple organic compound structurally similar to photosynthetic intermediates such as ribulose 1,5-bisphosphate, which plants can feed into the carbon-building steps of photosynthesis without going through CO2 ^[1]. Tropica describes its Carbon Nutrition as liquid carbon “in the easiest form for plants to absorb”, for stronger growth and to help prevent algae, without harming the aquarium’s microbiology at the recommended dose ^[3].

What the product pages do not say

Neither product page names its active compound ^{[1, 3]}. Seachem points to a safety data sheet on its SDS portal; that portal could not be read automatically when this page was prepared, so its contents are not reported here.

One thing Seachem does state, on a different product: the safety note for its trace fertilizer Flourish says it “contains 1,5-pentanedial” and may produce an allergic reaction ^[2]. 1,5-Pentanedial is another name for glutaraldehyde, whose IUPAC name in PubChem is pentanedial ^[4]. Whether Excel or Carbon Nutrition contain it is not stated on the pages cited here; check the safety data sheet supplied with the product.

Glutaraldehyde hazard data

For the pure substance, the aggregated GHS classifications in PubChem include: toxic if swallowed; causes severe skin burns and eye damage; may cause an allergic skin reaction; toxic or fatal if inhaled; may cause allergy or asthma symptoms if inhaled; very toxic to aquatic life, and toxic to aquatic life with long-lasting effects ^[4]. These classifications describe the substance itself. A dilute product’s actual hazard is stated on its own label and safety data sheet; Seachem’s Flourish label, for example, warns that it may produce an allergic reaction ^[2]. Keep such products away from children and follow the product’s own safety data sheet.

Seachem’s claims, as stated

The Excel page makes several specific claims. They are the manufacturer’s statements, reported here as such:

• Compared with CO2. “If CO2 is a 10, Flourish Excel is a 6-7”, and using both gives additional benefit ^[1].
• No pH effect. Excel “is not carbon dioxide and there is no impact on pH” ^[1], so it does not affect KH/pH-based CO2 estimates.
• Surface agitation does not reduce its effectiveness, according to Seachem, because it is not a dissolved gas ^[1].
• Iron. Seachem says Excel has iron-reducing properties that promote ferrous iron (Fe²⁺), which it describes as more easily used by plants ^[1].
• Dosing. 5 mL per 40 L on first use or after a large (>40 %) water change, then 5 mL per 200 L daily or every other day ^[1].

Algae: an observation, not a label claim

Seachem reports that hobbyists have observed less algae, black beard algae in particular, when dosing Excel, but states that it does not produce or market Excel as an algicide and that overdosing or applying it directly to plants as a spot treatment can cause damage ^[1]. Tropica, by contrast, does advertise its carbon product as helping prevent algae, through healthier plant growth ^[3]. See the black beard algae guide for what else is known about that group.

Plants that react

Seachem names two plants. Anacharis (Egeria) is described as particularly sensitive, and Seachem recommends dosing every other day rather than daily where it is kept ^[1]. For Vallisneria, Seachem reports its own trials: some leaf melt in a tank given double the dose, no problems at the recommended daily dose, and roots unaffected with strong regrowth afterward ^[1]. Seachem also says the majority of feedback it has received shows no negative effect on marimo balls at the directed dose, and suggests starting low with any sensitive plant ^[1].