A CO2 reactor kit gives you everything needed to dissolve CO2 into your planted tank in one package: the reactor body, fittings, and usually the connecting tubing. Some kits include a bubble counter or check valve as well. Buying a kit rather than piecing together components separately saves time and ensures the parts are matched for compatibility.

This guide covers what CO2 reactor kits typically include, how they differ from standalone reactors, the best kits available by tank size, and what you should add to a kit to build a complete functional system.

What a CO2 Reactor Kit Includes

A reactor kit is a bundled set of components centered on the reactor body itself. The exact contents vary by brand and model, but most kits include:

  • The reactor chamber (acrylic or clear plastic, with internal diffusion media or an impeller)
  • Inlet and outlet hose fittings sized for 12/16mm or 16/22mm tubing (depending on canister filter size)
  • Hose clamps or push-fit connectors
  • A CO2 inlet barb fitting
  • Sometimes a check valve and bubble counter

What kits almost never include is the regulator, cylinder, CO2 tubing, or canister filter. Those are sold separately. A reactor kit handles just the dissolution stage of the CO2 system.

Inline vs. External Reactor Kits

Inline reactor kits mount directly in the return line of a canister filter. The water from the filter flows through the reactor and then back into the tank. These are the most common type and the cleanest setup visually, since the reactor sits outside the tank on the cabinet.

External kits use a small submersible or external pump to push water through the reactor independently of the main filtration. These work on tanks without canister filters, like setups using hang-on-back or sponge filters. The tradeoff is slightly more equipment and tubing to manage.

Best CO2 Reactor Kits by Tank Size

Small Tanks (Under 40 Gallons)

For tanks under 40 gallons with low-flow canister filters (up to 400 L/h), the Rhinox CO2 Reactor 1000 is a compact inline option that comes with barbed fittings for 12/16mm tubing. At around $25-35, it is an affordable entry point. The acrylic body is clear, letting you monitor the CO2 gas pocket.

The Up Aqua CO2 Atomizer kits are another popular choice at this size. Rather than a true reactor, Up Aqua atomizers shred CO2 into very fine microbubbles using a pressurized chamber, achieving efficiency approaching a true reactor at lower cost. They work well on tanks under 30 gallons where CO2 demand is modest.

Medium Tanks (40-100 Gallons)

The CO2Art Inline CO2 Reactor in the standard size handles flow rates up to 1000 L/h, making it appropriate for Fluval 306, Eheim 2217, and similar mid-size canister filters. The kit includes the reactor body, 16/22mm inlet and outlet fittings, and CO2 barb fitting. It comes in around $50-60.

The Sera Flore CO2 Active Reactor 500 has been on the market long enough to accumulate a positive reputation. It handles up to 500 L/h and includes all necessary inline fittings. The construction quality is higher than most budget options, and replacement parts are available through Sera's distribution network. Price runs $50-65.

For a detailed comparison of CO2 reactor options across all sizes, check out our best CO2 reactor guide.

Large Tanks (100+ Gallons)

Tanks over 100 gallons need reactors that can handle high flow rates without air-locking. The CO2Art Pro Inline Reactor XL handles up to 1500 L/h and is one of the larger commercially available inline reactors. Alternatively, the Green Leaf Aquariums CO2 Reactor 500 is a well-regarded option in the planted tank community, with a larger body volume that provides more residence time for CO2 dissolution.

At this tank size, if you are running a canister filter with a flow rate above 1500 L/h (like a Fluval FX6), plan to run the reactor on a dedicated secondary pump rather than inline on the main return. A small Sicce or Eheim compact pump running 400-600 L/h handles the reactor loop while the main filter circulates the full tank volume.

What You Need to Complete a CO2 System

A reactor kit solves the dissolution problem, but you still need the upstream components to deliver CO2 to it.

CO2 Cylinder

For tanks up to 75 gallons, a 5lb aluminum cylinder is the practical choice. It connects to almost any dual-stage regulator via a CGA320 fitting (standard in the US). For larger tanks with higher injection rates, a 10lb cylinder extends time between refills. A 5lb cylinder runs roughly $40-60 new; 10lb cylinders run $60-90.

Dual-Stage Regulator with Solenoid

The regulator is where you spend a meaningful portion of your CO2 budget, and it is worth doing right. Dual-stage regulators prevent the end-of-tank dump problem that kills fish when single-stage regulators become erratic as the cylinder drains.

The CO2Art Pro-Elite Series dual-stage regulator includes a solenoid valve, working pressure gauge, and bubble counter in one unit for around $100-130. This is the cleanest single-purchase option for the regulator stage of a CO2 system. The Fzone CO2 Aquarium Regulator is a budget-friendly dual-stage option at around $60-75 that has earned positive reviews for reliable operation.

For a complete overview of CO2 systems from cylinder through diffusion, see our best CO2 system for aquarium guide.

Check Valve and CO2 Tubing

A check valve prevents water from back-siphoning into the regulator if pressure drops or the regulator is closed. Use a purpose-made aquarium CO2 check valve, not a standard airline check valve. CO2 check valves are rated for the pressures involved and use materials that do not degrade with CO2.

CO2 tubing (green 4/6mm or similar) has lower CO2 permeability than standard airline tubing. Long tubing runs (over 1 meter) with standard airline tubing can waste measurable amounts of CO2 through the tubing wall. CO2-rated tubing is inexpensive and worth using.

Assembling and Testing a CO2 Reactor Kit

Assembly takes about 20 minutes once you have all components.

Step-by-Step Installation

Turn off your canister filter and depressurize the return line. Cut the return outlet hose at a convenient point, leaving enough slack on each side to connect to the reactor fittings without tension. Push the inlet fitting onto the upstream hose end and the outlet fitting onto the downstream end. Secure both fittings with the included hose clamps, tightened just enough to prevent leaks without cracking the plastic fittings.

Connect the CO2 inlet tubing from your bubble counter output (or directly from the check valve) to the reactor's CO2 barb. Use a small hose clamp or wrap a loop of wire around the barb to ensure the tubing does not blow off under CO2 pressure.

Restart the canister filter. Water fills the reactor body. Slowly open CO2 flow from the regulator. Watch the gas pocket form in the upper part of the reactor. A healthy reactor shows a stable gas pocket occupying roughly 20-30% of the chamber. If it grows larger and does not stabilize, reduce CO2 flow.

Leak Testing

Run the system for 30 minutes and check all connection points by feel. A slight leaking CO2 tubing connection is hard to detect by smell but will be obvious by running your finger around the joint and feeling for the cold of escaping gas. Tighten connections or add small hose clamps as needed.

Common CO2 Reactor Kit Problems and Solutions

CO2 Microbubbles in the Tank

Small bubbles exiting the reactor into the return line and entering the tank are usually harmless to fish, but they indicate incomplete dissolution. This happens when the CO2 injection rate exceeds the reactor's capacity or the flow through the reactor is too fast. Reduce your bubble count or slow the flow through the reactor by partially closing the canister filter's output valve.

Reactor Air Lock

A full air lock occurs when the reactor body fills entirely with CO2 gas and water stops flowing. Reduce CO2 injection to well below the reactor's rated capacity and prime the reactor again by briefly disconnecting the CO2 line to let water refill the chamber. If air locks happen consistently, the reactor is undersized for your injection rate.

Drop in Canister Filter Flow

The reactor adds resistance to the return line and reduces overall filter flow. On well-rated canister filters, the flow reduction is typically 10-20%. If your flow drops more than that, check for a kinked return hose, an undersized reactor fitting, or calcium buildup inside the reactor body. A 50/50 white vinegar soak for two hours dissolves most calcium deposits.

FAQ

Does a CO2 reactor kit include the regulator? No. CO2 reactor kits include only the reactor body and its fittings. You need to purchase the CO2 cylinder, regulator, solenoid, check valve, and CO2 tubing separately. Some kits include a bubble counter and check valve as extras, but the regulator and cylinder are always separate purchases.

What is the difference between a CO2 reactor and a CO2 diffuser? A diffuser releases CO2 as fine bubbles into the water column, and some of those bubbles escape at the surface before dissolving. Dissolution efficiency runs 60-80%. A reactor completely encloses CO2 in a turbulent water chamber until it dissolves, achieving near-100% efficiency. For tanks over 40 gallons, the efficiency advantage of reactors over diffusers translates to meaningful CO2 savings.

Can I use a CO2 reactor kit with a hang-on-back filter? Not inline, since hang-on-back filters do not have external hose connections. You can run a CO2 reactor off a dedicated small powerhead installed in the tank. Mount the reactor externally and run tubing from the powerhead through the reactor and back into the tank. This works well but adds one more component to the setup.

How do I clean a CO2 reactor? Soak the reactor body in a 50/50 solution of white vinegar and water for 1-2 hours every few months. This dissolves calcium deposits that accumulate inside the chamber and on the inlet/outlet fittings. Rinse thoroughly with tap water before reinstalling. Do not use bleach on acrylic reactor bodies as it may cloud the material.

Summary

A CO2 reactor kit gives you the reactor body and fittings needed to integrate inline CO2 dissolution into a canister filter return line. Pair it with a 5lb CO2 cylinder, a dual-stage regulator with solenoid, a check valve, and CO2-rated tubing to build a complete system. For tanks under 40 gallons, the Rhinox 1000 or Up Aqua atomizer kits are practical and affordable. For medium tanks, the CO2Art Inline Reactor or Sera Flore 500 are well-regarded options with solid long-term track records. The complete system cost for a medium planted tank runs $130-200, with most of that in the regulator and cylinder rather than the reactor itself.