“LIquid CO2” or “liquid carbon” is a liquid (naturally) that is advertised to provide a usable and growth-impactful source of carbon to submerged plants. Although it’s hard to get definitive component analysis, liquid carbon is inevitably either glutaraldehyde, or a derivative form of glutaraldehyde e.g. a cyclised variant. The alternative to liquid carbon is to directly add carbon dioxide gas to the water. Whether liquid carbon actually works, and if so how, is a subject of considerable debate.
One reasonably definitive consideration is that liquid carbon is not converted to meaningful levels of carbon dioxide in the water column. The concentation of dissolved carbon dioxide is fairly easy to measure using a drop checker. The drop checker is a blue colour when CO2 levels are too low, green when they are right and yellow when they are too high. Despite careful observation, I have never seen liquid carbon affect the colour of a drop checker, and nor has anyone else near as I can tell. So that’s settled then.
The next place the argument in favour of liquid carbon generally goes is that the carbon-containing glutaraldehyde molecules in the liquid carbon, whilst not themselves directly usable as a carbon source by plants, can be converted by aerobic bacteria into molecules which can be taken up and used metabolically by plants. It is much more difficult to get to either the truth or falsity of this argument. That being said, if plants could usefully take up soluble carbon source molecules, then why not simply provide those molecules directly as “liquid carbon” rather than requiring some mysterious bacterial metabolic process on glutaraldehyde? Glutaraldehyde is known toxic and used as a hospital disinfectant so use in aquariums has been hugely controversial. Why take on all this controversy if less toxic plant-accessible carbon metabolic precursors could be provided?
Liquid carbon is an algaecide
Glutaraldehyde, consistent with its disinfectant properties, is a known algaecide. Usefully, algae is (in general) more sensitive to glutaraldedhyde toxicity than are plants, fish, shrimp, snails and other desireable residents of an aquarium. This means if you can get the “liquid carbon” concentration right, you can suppress algae growth. I think the suppression of algae is why liquid carbon is useful. Indirectly, algae suppression can benefit plant growth, both because the algae will no longer be competing with plants for nutrients, and because algae tends to grow on upper plant leaf surfaces where it competes for light. When this doesn’t happen, plants grow better. I use liquid carbon regularly, and algae suppression is the reason why. I use 1 ml liquid carbon every day in my 45 L tank.
Spot treating algae with liquid carbon
You can also spot treat algae problems by treating with undiluted liquid carbon. The undiluted form is well-capable of sterilization so as long as the undiluted form doesn’t contact the livestock you’re good. I take advantage of this during water changes. Since I’m using estimative index fertilizers, I change out 50% of the water weekly. After removing the water the surfaces of the hardscape components of the aquarium closest to the light, where the algae grows most vigorously, are exposed. On water change days I measure out 1.5 ml liquid carbon and spot treat that around on those surfaces. When the new water is added back, the concentration of the liquid carbon reverts to about where you’d like it to be (maybe slightly higher). This spot-treatment has been particularly effective in clearing the algae from the central bubble tube of the aquarium, which otherwise can be very problematic to clean out.
Added note: also works brilliantly to clear up scuzzy CO2 diffusers!