Guide
CO2 additive: what you need to know
Liquid CO2, Excel, liquid carbon - what these products actually do and when they help.
- Not actual CO2: Liquid CO2 products like Seachem Excel, Easy Carbon, and API CO2 Booster are glutaraldehyde-based biocides - the "CO2" label is marketing, not chemistry.
- Modest carbon boost: Some plants can metabolise glutaraldehyde-derived carbon, giving a small growth improvement in low-tech, low-light tanks.
- Algae control is the real selling point: Glutaraldehyde is an algaecide. It's genuinely useful against hair algae and black beard algae at correct doses.
- They don't replace injected CO2: For carpeting plants, red plants, or any high-tech aquascape, you need a pressurised CO2 system. Additives can't deliver comparable carbon availability.
- Dose carefully: Overdosing damages plants, stresses fish, and can kill shrimp and snails. Certain plant species react badly even at normal doses.
What "CO2 additive" actually means
The name does a lot of misleading work. When you buy a bottle labelled "liquid CO2," you're not buying dissolved carbon dioxide gas. You're buying a dilute solution of glutaraldehyde - the same class of compound used as a hospital disinfectant and as a preservative in biological specimens. At aquarium doses it's far less concentrated, but the chemistry is the same.
Glutaraldehyde is a biocide, which means it kills or inhibits microbial and algal cells. That's actually the mechanism behind both its algae-control properties and its carbon-source claims. Some aquatic plants, particularly fast-growing stem plants and certain mosses, can break down glutaraldehyde and use the resulting carbon compounds for growth. Others can't, or don't noticeably respond to it.
The marketing term "liquid CO2" has stuck because it's easier to sell than "glutaraldehyde solution," but don't let it shape your expectations. The two things are not interchangeable in any meaningful biological sense.
The CO2 additive category exists partly because pressurised CO2 systems intimidate beginners and take up space. These products fill a commercial gap more than a biological one. They can be useful tools, but only if you understand what they actually do - not what the name implies.
What CO2 additives actually do in your tank
Algae control
This is where these products genuinely earn their place. Glutaraldehyde disrupts cell membranes and inhibits photosynthesis in algae at concentrations that most fish and plants tolerate. Spot-treating black beard algae (BBA) with a few drops applied directly via a syringe - flow off, wait 20-30 seconds, then resume circulation - is a widely used technique that works. Hair algae outbreaks in low-tech tanks also respond well to consistent dosing at label-recommended amounts.
The algaecide effect is real and repeatable. If algae control is your primary goal, these products are a legitimate option alongside addressing the root cause - usually excess light or a nutrient imbalance.
Carbon supplementation for plants
The growth-boosting effect is real, but modest and plant-dependent. In a low-tech tank with low to medium light, adding a CO2 supplement can nudge growth rates upward for species that respond to it - certain stem plants, java moss, and fast-growing floating plants tend to show the clearest response. For slow-growing, undemanding species like Anubias barteri, Microsorum pteropus (java fern), or Bolbitis heudelotii, the benefit is usually negligible. Those plants grow slowly regardless, and they're not limited primarily by carbon availability.
The honest comparison: injected CO2 at 20-30 mg/L can roughly double or triple plant growth rates and enable a completely different tier of plant species. Glutaraldehyde additives, even used consistently, don't get close to that effect.
What they don't do
CO2 additives don't raise dissolved CO2 levels in your water column in any meaningful or measurable way. They don't affect pH the way real CO2 injection does (injected CO2 forms carbonic acid, predictably lowering pH). They don't enable carbon-hungry carpeting plants like Hemianthus callitrichoides (HC Cuba) or Glossostigma elatinoides to thrive the way a pressurised system does. If your plant keeps melting or refuses to carpet, the additive isn't the fix.
Assuming more is better. Doubling the dose doesn't double the benefit - it damages leaves, melts sensitive plants, stresses fish, and can kill the entire shrimp population in a tank overnight. The label dose is already set near the upper edge of what's safe.
When a CO2 additive makes sense
There's a specific type of tank where these products are genuinely useful: a low-tech planted setup with low to medium light (under 30-40 PAR at the substrate), undemanding or moderately demanding plants, and a keeper who doesn't want the complexity or cost of a pressurised CO2 system.
In that scenario, a CO2 supplement can help keep opportunistic algae in check while providing a small carbon boost to more responsive plant species. It's not transformative, but it's a real, low-effort tool. Combined with good fertilisation - both macro and micro nutrients - and appropriate light duration (6-8 hours for low-tech tanks), you can maintain a healthy, attractive planted tank without cylinders, regulators, or diffusers.
It also makes sense as a spot treatment for algae outbreaks regardless of your CO2 setup. Even experienced aquarists with injected CO2 keep a bottle of Excel on hand for targeted BBA treatment.
When you need real CO2 injection instead
If your plant list includes anything that demands high carbon availability, stop looking at additives and start pricing CO2 systems. The plants that consistently fail without injected CO2 include:
- Carpeting species: Hemianthus callitrichoides, Glossostigma elatinoides, Eleocharis acicularis (hairgrass in dense carpet form), Marsilea hirsuta
- Most red and orange stem plants: Rotala macrandra, Ludwigia arcuata, Alternanthera reineckii 'Mini'
- Fast-growing stems in general: Rotala rotundifolia, Hygrophila pinnatifida, Didiplis diandra
These plants can survive without CO2 but won't thrive, carpet, or hold their colour. High light without CO2 injection is one of the most reliable ways to grow algae rather than plants - the light drives photosynthesis faster than the plants can process available carbon, and algae fills the gap.
If you're planning a high-tech aquascape, the CO2 system should come before the demanding plants. Get your injection running, dial in your bubble rate (target 20-30 mg/L dissolved CO2), and confirm stability before stocking the harder species.
Safety, dosing, and what can go wrong
General dosing
Follow the manufacturer's label. Most products recommend dosing once daily, typically 1 ml per 50-100 litres (10-25 US gallons), though this varies by brand. Dose after lights on when plants are actively photosynthesising - don't dose into a dark tank where plants aren't consuming it.
Sensitive species: plants
Vallisneria species are notoriously reactive to glutaraldehyde. Even standard doses cause vallis leaves to melt, sometimes completely. If you're keeping any vallis - Vallisneria spiralis, V. americana, giant vallis - avoid these products entirely or risk losing the plants. Elodea (Canadian pondweed) and some other fine-leafed oxygenators can also react badly.
Sensitive species: invertebrates
Shrimp and snails are significantly more sensitive to glutaraldehyde than fish are. In a shrimp-only or planted shrimp tank (Neocaridina davidi, Caridina cantonensis, nerite snails), standard doses can cause stress and death. If you keep inverts, start at 25-50% of the recommended dose and watch for behaviour changes - shrimp suddenly becoming hyperactive or climbing the glass are warning signs. Many shrimp keepers avoid CO2 additives altogether and use alternative algae management strategies.
Shrimp deaths from CO2 additive overdose are often delayed by 12-24 hours, so you may not see the consequences until the next morning. If you're in doubt about your dose, err well below the label amount in any tank with invertebrates.
Fish sensitivity
Most fish tolerate label-dose glutaraldehyde without obvious stress at normal aquarium concentrations. That said, labyrinth fish, small rasboras, and nano species that naturally inhabit very soft, clean water may show more sensitivity. If you've just added new fish or your tank is already under any stress - a cycling issue, temperature swing, disease - this isn't the moment to introduce a new chemical. Check your water parameters are stable first; the aquarium water parameters guide covers what "stable" looks like in practice.
CO2 additives vs. other approaches to plant growth
Before reaching for a supplement, make sure you're not neglecting the factors that matter more. Most planted tank problems come down to light duration and intensity, macronutrient availability (nitrogen, phosphorus, potassium), and micronutrient availability (iron, trace elements) - in that order. A CO2 additive won't compensate for a fertiliser deficit or a light running 12 hours a day and fuelling algae.
A practical low-tech planted tank approach that works:
- 6-8 hours of light at low to medium intensity (15-30 PAR at the substrate for undemanding plants)
- A comprehensive liquid fertiliser dosed 2-3 times weekly, or a nutrient-rich substrate
- Regular water changes - 25-30% weekly - to export waste and replenish minerals
- A CO2 supplement if algae is a recurring problem or you want a modest growth boost
That combination will outperform any amount of additives on a neglected or incorrectly lit tank.
This is the range most planted tank aquarists aim for with pressurised CO2 - a concentration that genuinely accelerates plant growth and that no liquid additive comes close to replicating.
Choosing a product
The market leaders - Seachem Excel, Easy Carbon (Tropica), and API CO2 Booster - all use glutaraldehyde or a polycycloglutaracetal derivative as the active ingredient and perform similarly. Seachem Excel has the longest track record and the most user data behind it. Tropica Easy Carbon is slightly more concentrated per ml. Own-brand versions from aquarium retailers are usually the same chemistry at lower cost.
There's no meaningful reason to pay a premium between brands. Buy whichever is available locally, dose conservatively, and keep the bottle away from your vallis.
Track your parameters and get care reminders with App-aquatic.
Get the free appDoes liquid CO2 actually add CO2 to my tank water?
No. Liquid CO2 products contain glutaraldehyde, not dissolved carbon dioxide. They don't raise CO2 levels in your water column or affect pH the way injected CO2 does. Some plants can extract a carbon source from glutaraldehyde, but it's a fundamentally different process from CO2 injection.
Can I use a CO2 additive in a shrimp tank?
With caution and a reduced dose. Shrimp are significantly more sensitive to glutaraldehyde than fish. Start at 25-50% of the label dose and observe behaviour closely. Many shrimp keepers skip these products entirely and manage algae through other means - light control, manual removal, algae-eating species - to avoid the risk.
Will CO2 additives kill my vallisneria?
They can, yes. Vallisneria is well-documented as highly sensitive to glutaraldehyde. Standard doses often cause the leaves to melt. If you keep any vallis species, avoid these products or accept you may lose the plants.
How do CO2 additives help with black beard algae?
Glutaraldehyde is an algaecide and directly damages BBA cells. Spot-treating affected areas - turn off flow, apply a few drops directly to the algae via syringe, wait 20-30 seconds, then restore flow - is an effective technique. The algae typically turns red or pink within a day or two and then dies off. Consistent daily dosing also suppresses regrowth, but you still need to address the underlying cause (usually excess light or phosphate).
I'm using a CO2 additive but my plants still aren't growing well. What am I missing?
Almost certainly fertiliser or light - or both. Carbon isn't the only thing plants need, and in a low-tech tank it's often not the primary limiting factor. Check whether your light is appropriate for the plants you're keeping, ensure you're dosing both macro and micro nutrients regularly, and confirm your tank is fully cycled. The fish tank cycling guide and water parameters guide are good starting points if you're unsure about the basics.
More guides · Plant fertiliser · Beginner plants · Black beard algae