Is liquid carbon safe for shrimp? Glutaraldehyde, CO2 and shrimp tanks
Planted-tank keepers add carbon to grow plants faster, either as a liquid "carbon" supplement or as CO2 gas. Both are common in shrimp tanks, and both can kill shrimp when overdone. This guide explains what each one is, where the risks lie and how keepers who use them keep shrimp safe. Most shrimp tanks with mosses, ferns and other easy plants don't need either.
Do shrimp tanks need added carbon?
No. Shrimp don't need it, and the plants shrimp keepers favor, such as Java moss, Java fern and Anubias, grow fine without it under modest light; see Java moss and other mosses and best plants for shrimp tanks. Added carbon makes sense mainly in brightly lit, densely planted tanks where plants run short of it. Practical Fishkeeping notes that plants short of CO2 can take carbon from the water's carbonate hardness, which can leave chalky deposits on leaves.
What liquid carbon is
Practical Fishkeeping describes the liquid carbon products sold for planted tanks as based on glutaraldehyde, a sterilizing chemical also used for dental equipment, wart treatments and leather tanning. It warns: "Overdose it and it'll ravage fish and invertebrates." It also notes that some plants react badly; for example, Phoenix moss (Fissidens) reacts badly to high concentrations, according to its moss guide.
Liquid carbon and shrimp: where the risk is
- Overdosing. Glutaraldehyde-based products are often reported by keepers to stress or kill shrimp when overdosed. If you use one, measure carefully and stay at or below the label's dose for your real water volume, which is smaller than the tank's rated size once substrate and hardscape are in.
- Spot-dosing algae. Squirting liquid carbon directly onto algae concentrates it, and shrimp owners commonly report losses after doing this. Our algae guide covers shrimp-safe ways to deal with algae.
- Combining with other additives. Keep algaecides, pesticides and copper out of a shrimp tank entirely; see shrimp-safe fertilizer and is copper toxic to shrimp.
Many shrimp keepers simply skip liquid carbon. If you do try it, start well under the dose, watch the shrimp closely for a few days and stop at the first sign of unusual behavior; see why shrimp swim frantically.
Pressurized CO2 and shrimp
CO2 gas dissolves into the water and forms carbonic acid, which lowers pH. Too much of it makes it harder for fish and shrimp to breathe. Practical Fishkeeping calls overdosing CO2 the biggest killer of fish and shrimp in the planted tank hobby and probably the biggest cause of livestock deaths in planted tanks.
| Level | What Practical Fishkeeping says |
|---|---|
| About 15 ppm | Suggested for low-energy planted tanks |
| 10–25 ppm | The range many experts quote; Amano recommended 15–25 ppm |
| About 30 ppm | Safe for most fish and invertebrates, though the threshold depends on oxygen; the author never goes over it |
| Above 30 ppm | Increasing risk; some manufacturers quote 35–40 ppm, but overdose is the main danger |
Practical Fishkeeping adds that CO2 should stay steady during the lit period, because swings encourage algae, and that pressurized cylinders hold gas at about 1,000 psi and should be kept away from children and pets. It warns that cheap kits can be dangerous and that yeast-fermentation kits can't be switched off.
How to measure CO2
- Drop checker. A small glass bulb filled with a 4 dKH reference solution and bromothymol blue sits in the tank. Practical Fishkeeping says lime green means about 30 ppm, blue less and yellow more, and that it takes several hours to respond, so it shows the trend rather than the moment. For low-energy tanks it suggests diluting the 4 dKH solution 50% with RO water to target about 15 ppm.
- pH and KH charts can mislead. Practical Fishkeeping notes that small test errors (0.2 pH or 0.5 KH) can mean the real level is 45 ppm or more, and that other acids and phosphate buffers distort the reading.
Because CO2 lowers pH through the day, expect a regular daily pH swing in an injected tank; see shrimp tank pH.
Keeping shrimp safe with CO2
- Turn CO2 off at night. Plants stop using CO2 in the dark but still release it, so Practical Fishkeeping switches injection off at night, usually with a solenoid on a timer.
- Check late and early. It suggests watching livestock late at night and early in the morning, when CO2 is highest.
- Keep oxygen up. Surface agitation drives off excess CO2. Practical Fishkeeping notes high CO2 interferes with how fish carry oxygen in the blood, and less oxygen narrows the safe margin.
- Add new livestock carefully. Practical Fishkeeping turns off lights and gas for 24 hours before adding new fish, and the same caution suits new shrimp; see how to drip acclimate shrimp.
- Use the right hardware. A pressurized system with a solenoid, needle valve, bubble counter and non-return valve gives control that yeast kits don't.
If shrimp are gasping
Shrimp crowding at the surface or the filter outlet, or swimming restlessly, can be a sign of too much CO2 or too little oxygen. Practical Fishkeeping's emergency steps:
- Turn off the CO2.
- Add an air stone or point the filter outlet at the surface to drive off CO2.
- Do a large water change to bring CO2 down quickly, with water matched for temperature and hardness.
Then reduce the bubble rate and let the drop checker settle for several hours before adjusting again. Our quiet air pump guide covers air pumps that suit shrimp tanks.
Track it
Log your bubble rate, drop checker color and pH on the free Water Parameter Log printable, so any change shows up before the shrimp react to it.
Frequently asked questions
Is liquid carbon safe for shrimp?
It is risky. Most liquid carbon products are glutaraldehyde-based, and Practical Fishkeeping warns overdosing ravages invertebrates. Many shrimp keepers skip it; if you use it, stay at or below the label dose.
Can I spot-dose liquid carbon on algae in a shrimp tank?
It is best avoided. Spot-dosing concentrates glutaraldehyde, and shrimp owners commonly report losses after doing it. Remove algae by hand and fix light and nutrients instead.
Is CO2 injection safe for shrimp?
Yes when controlled. Practical Fishkeeping treats about 30 ppm as safe for most invertebrates, but overdosing is the biggest killer of fish and shrimp in planted tanks.
What color should my drop checker be for shrimp?
Lime green, which with a 4 dKH solution means about 30 ppm. Yellow means too much. Practical Fishkeeping suggests aiming slightly darker green for a safety margin.
Should CO2 be turned off at night in a shrimp tank?
Yes. Plants stop using CO2 in the dark but still release it, so levels rise overnight. Use a solenoid on a timer.
What do I do if my shrimp are gasping at the surface?
Turn off the CO2, add an air stone or point the filter outlet at the surface, and do a large matched water change.
Do shrimp tanks need CO2?
No. Shrimp don't need it, and mosses, Java fern and Anubias grow well without it under modest light.
Sources: Practical Fishkeeping, Be clear about carbon (liquid carbon based on glutaraldehyde, a steriliser used for dental equipment, wart cures and leather tanning; "Overdose it and it'll ravage fish and invertebrates"; too much CO2 can drop pH dangerously; budget CO2 kits can be dangerous; CO2-starved plants may take carbon from carbonate hardness, leaving chalky deposits); Practical Fishkeeping, CO2: striking the balance (CO2 highly toxic when overdosed and probably the biggest cause of livestock deaths in planted tanks; 30 ppm safe for most fish and inverts, depending on oxygen; pH/KH table errors can mean 45 ppm or more; drop checker with 4 dKH solution and bromothymol blue, lime green about 30 ppm, aim darker green, slow to change; about 15 ppm for low-energy tanks with 4 dKH solution diluted 50% with RO; CO2 off at night; watch fish late at night and early morning; lights and gas off 24 hours before adding fish; large water changes lower CO2 in emergencies); Practical Fishkeeping, What CO2 can do for you (and your plants!) (overdosing is the biggest killer of fish and shrimp in the planted tank hobby; keep CO2 stable while lights are on; cylinders about 1,000 psi, away from children and pets; yeast kits uncontrollable; solenoid, needle valve, bubble counter and non-return valve); Practical Fishkeeping, Frequently asked questions on adding carbon dioxide (CO2 forms carbonic acid and lowers pH; switch off at night; if fish gasp, turn off the gas and add an air stone or point outlets up; experts quote 10–25 ppm, Dennerle 35–40, Amano 15–25; high CO2 inhibits oxygen transport in blood; phosphate buffers distort the KH–pH–CO2 relationship); Practical Fishkeeping, Five of the best aquarium mosses (Phoenix moss reacts badly to high concentrations of liquid carbon). Reports of shrimp losses after liquid carbon overdosing or spot-dosing are recurring owner reports.