Used aquarium CO2 systems are worth buying in some cases and worth avoiding in others, depending entirely on which components you're looking at. Regulators, diffusers, drop checkers, and tubing can all be purchased used with relatively low risk. CO2 cylinders (the pressurized tanks) have more considerations around testing dates and valve condition. The CO2 regulator itself, the most expensive single component in a pressurized system, is generally fine to buy used if it's from a reputable brand and has been tested. Budget $60-$120 for a used quality regulator versus $120-$250 new; that's a real savings on a piece of gear that can last 10-15 years with basic maintenance.

This guide covers where to find used CO2 systems, what each component is worth buying secondhand versus new, how to evaluate used regulators and cylinders, and what fair prices look like for complete setups versus individual components.

What a Complete Aquarium CO2 System Includes

Before shopping used, it helps to know what a full pressurized CO2 system consists of, because sellers often list systems at different levels of completeness.

A complete pressurized system includes: - CO2 cylinder: The pressurized tank holding the CO2 gas. Common sizes are 5 lb, 10 lb, and 20 lb aluminum or steel cylinders. - Regulator: The device that reduces high-pressure CO2 from the cylinder to a low, controllable output pressure. Quality regulators have dual gauges (working pressure and tank pressure) and a solenoid for timed operation. - Needle valve: Allows fine adjustment of CO2 flow rate. Either integrated into the regulator or a separate component. - Solenoid valve: An electrically-operated valve that opens when powered (usually connected to an outlet timer) to stop CO2 injection at night when plants aren't photosynthesizing. - CO2-rated tubing: Standard aquarium airline tubing is too porous for CO2; you need silicone or specialized CO2 tubing. - Diffuser or reactor: The component that dissolves CO2 into tank water. Glass diffusers (like the UP Aqua 45mm, $10-$20) create fine bubbles; CO2 reactors dissolve CO2 inline for higher efficiency. - Drop checker: A glass device filled with a pH-sensitive indicator solution that shows CO2 concentration in the tank water by color (blue = low, green = optimal, yellow = excessive). - Bubble counter: Counts the rate of CO2 injection in bubbles per second.

A complete used system from a reputable seller running the full setup can be purchased for $150-$300, compared to $350-$600 for a complete new system using equivalent quality components.

The CO2 Regulator: Key Component and Best Used Buy

The regulator is where most of the value in a used CO2 system lies. New quality regulators from GLA (Green Leaf Aquariums), CO2Art, Aquatek, and the Milwaukee MA957 run $120-$250. Used units from these brands in working condition sell for $60-$130.

What to Check on a Used Regulator

Solenoid function: Connect it to a 12V DC power source (or the appropriate voltage for the specific unit) and verify the solenoid clicks open when powered and closes when power is removed. A quiet or sticky solenoid may need a cleaning or replacement; replacement solenoids cost $15-$30.

Gauge accuracy: Tank pressure gauges on new 5 lb cylinders should read around 850-900 PSI at room temperature. A gauge showing a dramatically different reading when tested with a known-good cylinder suggests calibration drift or damage.

O-ring and seal condition: Inspect the regulator's outlet port and CGA-320 inlet connector (the part that attaches to the cylinder) for cracked or missing O-rings. Replacement O-ring kits are inexpensive ($3-$8), but a regulator with a leaking connection at the cylinder is a safety issue.

Needle valve sensitivity: A quality needle valve allows very small adjustments in bubble rate. If the needle valve is stiff, difficult to turn, or jumps from zero flow to a heavy flow with no middle range, it may be gummed with mineral deposits or worn. Soaking in white vinegar sometimes fixes this; replacement needle valves cost $15-$40.

Our dedicated guide to best CO2 systems for aquariums covers current new options if you want to compare against used pricing.

CO2 Cylinders: More Considerations Than Most Buyers Expect

CO2 cylinders are subject to federal DOT hydrostatic testing requirements in the US. Every aluminum and steel cylinder must be tested every 5 years to verify structural integrity. The test date is stamped on the collar of the cylinder (format: month/year). A cylinder with an expired test date cannot be legally refilled at most welding supply shops, paintball stores, or fire extinguisher service centers until it's retested.

Retesting a 5 or 10 lb cylinder costs $15-$30 at a welding supply shop and takes 1-3 days. Factor this into your evaluation when buying a used cylinder with an expired date.

Cylinder Pricing

Used CO2 cylinders are priced mainly by size and condition:

  • 5 lb aluminum cylinder (empty): $20-$45 used, $60-$80 new
  • 10 lb aluminum cylinder (empty): $30-$55 used, $80-$110 new
  • 20 lb aluminum cylinder (empty): $40-$70 used, $100-$140 new

A steel cylinder of the same size typically runs $10-$20 less than aluminum but weighs more.

Cylinders with a test date within the past 3 years are worth more because you get 3+ years before the next test. Cylinders within 1 year of the next test date should be priced accordingly or retested before the sale.

Where to Find Used CO2 Systems

Planted Tank and Reef Forums

The Planted Tank forum (plantedtank.net) has an active classifieds section specifically for planted tank equipment, including CO2 systems. Sellers are experienced hobbyists who usually describe condition accurately. This is one of the best sources for used GLA and CO2Art regulators.

REEF2REEF classifieds also has CO2 equipment listings, as reef hobbyists use CO2 for calcium reactors.

Facebook Marketplace and Craigslist

Search "aquarium CO2 regulator," "planted tank CO2 kit," or specific brand names. Local pickup allows you to test the regulator before buying, which is worth the effort for a component in the $80-$150 price range.

eBay

eBay has consistent CO2 equipment listings. For cylinders, prioritize local pickup or be prepared for freight shipping costs on anything over 10 lbs. For regulators and accessories, standard shipping is fine.

What's Fine to Buy Used vs. Buy New

Buy used: Regulators (from quality brands, tested), CO2 cylinders (check the test date), glass diffusers (inspect for chips), drop checkers, bubble counters, stands, and accessories.

Buy new: CO2-rated silicone tubing (degrades and becomes porous over time, cheap to replace at $5-$10 for 6 feet), solenoid valve if there's any doubt about seating or condition, needle valves if the existing one shows wear.

For CO2 reactors, used is generally fine. The UP Aqua CO2 Turbo Atomizer and the Rhinox Glass CO2 Reactors are glass units worth checking carefully for chips or cracks at the connection points. See our guide to the best CO2 reactors for comparisons.

Pricing a Complete Used CO2 System

Here's what fair pricing looks like for common complete setups:

Budget complete kit (dual-gauge regulator, no solenoid, glass diffuser, 5 lb cylinder): New equivalent: $130-$180. Used: $60-$100.

Mid-range complete kit (quality regulator with solenoid, needle valve, drop checker, 10 lb cylinder): New equivalent: $280-$380. Used: $130-$200.

Premium setup (GLA or CO2Art regulator, inline reactor, all accessories, 20 lb cylinder): New equivalent: $450-$600. Used: $200-$320.

If a seller prices a used setup above 70% of the current new price without documentation of recent service or a test, counter-offer or pass.

FAQ

Is it safe to buy a used CO2 regulator for my aquarium? Yes, from reputable brands with proper testing. CO2 regulators from GLA, CO2Art, Aquatek, and Milwaukee are built for long service life and hold up well secondhand. The main risks are worn seals and degraded needle valves, both of which are cheap to address. Test the solenoid and check for leaks at all connections before and after installation.

How do I know if a used CO2 cylinder is still valid? Look at the test date stamped on the collar. It's formatted as month/year (for example, "06/21" means the hydrostatic test was performed in June 2021, and the next test is due in June 2026). If the next test date is within a year, budget for a retest at a welding supply shop.

Can I refill a used CO2 cylinder anywhere? Welding supply shops, paintball stores, homebrew shops, and fire extinguisher service centers all fill CO2 cylinders. Most require the test date to be current. Some paintball and homebrew shops are more lenient; welding supply shops almost always check. A cylinder exchange program at places like AirGas or Praxair is the simplest option if you want to avoid waiting.

What's the best brand of used CO2 regulator to look for? GLA (Green Leaf Aquariums) regulators are the most highly regarded in the planted tank community and hold their value well. CO2Art Pro SE and Pro Dual Stage regulators are also excellent. The Milwaukee MA957 is a widely available and reliable entry-level regulator that frequently appears on the used market at $40-$70.

Conclusion

Buying a used CO2 system is one of the best ways to save money on planted tank equipment. Regulators from quality brands, tested and confirmed functional, represent the biggest cost savings. Cylinders require a date check but are otherwise straightforward. Focus on sellers who describe condition specifically, test regulators before you buy if at all possible, and buy new tubing and seals regardless. A complete used mid-range CO2 setup in good condition runs $130-$200, about half the cost of assembling the same system new.