An in-tank CO2 reactor is a device that dissolves carbon dioxide gas directly inside your aquarium water column, rather than outside in a canister or inline unit. You submerge it in the tank, hook it up to your CO2 line, and the reactor mixes the gas with tank water until nearly 100% of the CO2 dissolves before any bubbles escape. For planted aquariums, this is one of the most efficient ways to get CO2 where your plants actually need it.
This guide covers how in-tank reactors work, how they compare to other CO2 diffusion methods, the best setups for different tank sizes, and the common mistakes people make when using them. If you've been losing CO2 bubbles to the surface or your plants still look nutrient-starved even with a system running, the problem might be your diffusion method.
How an In-Tank CO2 Reactor Works
The basic principle is simple. CO2 gas enters a sealed chamber that is submerged in your aquarium. A small impeller or pump pulls tank water into the chamber, creating turbulent mixing between the gas and liquid. The CO2 dissolves into the water inside the chamber. When water exits the reactor, it carries dissolved CO2 with it. No gas bubble escapes into the tank unless the chamber is overfilled.
This is different from a ceramic diffuser, which releases tiny bubbles into the water column and relies on those bubbles dissolving before they reach the surface. Diffusers are less efficient because some bubbles always escape. A well-designed in-tank reactor can hit 95-99% dissolution rates under normal conditions.
The Role of the Impeller
Most in-tank reactors use a small pump to circulate water through the mixing chamber. Some are "passive" designs that attach directly to a filter outlet and use the filter's existing flow. The active pump models give you more control over flow rate, which directly affects how fast CO2 dissolves and how evenly it distributes.
The Aquario Neo CO2 Reactor and the Up Aqua CO2 Reactor are two widely used in-tank models. Both use passive flow designs that attach to a powerhead or filter outlet. They work well in tanks under 50 gallons when paired with a pump that moves at least 50 gallons per hour.
In-Tank vs. Inline CO2 Reactors
This is the most common comparison you'll see, and the answer depends on your setup.
Inline reactors sit outside the tank on your external filter's return line. Water passes through them before re-entering the aquarium. They're completely hidden from view, which is a big advantage for display tanks. They also tend to dissolve CO2 extremely well because the flow rate is controlled by your canister filter, which typically runs at a steady 150-300+ GPH.
In-tank reactors are visible inside the aquarium. This is the main trade-off. However, they work without an external canister filter, which makes them accessible for anyone running a hang-on-back filter or sponge filter. They're also easier to install and maintain since you don't need to cut into plumbing.
If you have a canister filter and care about aesthetics, inline is the cleaner option. If you're running an internal or HOB filter, an in-tank reactor is the most efficient CO2 diffusion method available to you short of switching to a full external setup.
Size Matching Matters
A common mistake is pairing a small reactor with a large tank or a powerful pump with a small reactor. Most in-tank reactors are rated for specific tank sizes. The Up Aqua CO2 Reactor Mini is rated for tanks up to 30 gallons. The standard size handles up to 60-75 gallons. If you push too much flow through a small reactor, the residence time inside the chamber drops, and CO2 escapes before dissolving.
Installation and Placement Tips
Placement inside the tank affects both CO2 distribution and visual impact.
Put the reactor near the outlet of your circulation pump or powerhead so that dissolved CO2-rich water gets spread throughout the tank quickly. Placing it in a dead zone where water stagnates means your plants on the other side of the tank might not see adequate CO2 levels even when your drop checker looks green.
Most in-tank reactors mount with suction cups on the glass. Position it somewhere accessible for cleaning, because the impeller and chamber will accumulate algae and biofilm over time.
The outlet of the reactor should point toward open water, not directly at the substrate or toward a wall. You want the dissolved CO2 to circulate, not pool in one corner.
Dealing with Gas Buildup
Every in-tank reactor accumulates a small gas pocket at the top of the chamber over time. This is normal. The trick is managing it. If the gas pocket gets too large, it reduces the water volume inside the chamber and makes dissolution less efficient. Some reactors have a small bleed screw or vent at the top. Others require you to tilt the unit to burp the gas out through the outlet.
Check the gas pocket weekly when you're first getting calibrated. Once you know how fast it builds up, you can add a quick purge to your maintenance routine.
CO2 Bubble Rate and Adjustments
Getting your CO2 bubble rate right with an in-tank reactor is slightly different from using a diffuser.
With a diffuser, you can see the bubbles and adjust accordingly. With a reactor, the bubbles disappear inside the chamber. You need to rely on a drop checker or CO2 indicator solution to know where your CO2 level sits.
Target a light green color on a standard drop checker using 4 dKH reference solution. That corresponds to roughly 30 parts per million dissolved CO2, which is in the sweet spot for most planted tanks. Yellow means too much (above 40 ppm, which starts stressing fish). Blue means too little (below 15 ppm, which limits plant growth).
Start with 1-2 bubbles per second and adjust from there. Large, heavily planted tanks with fast-growing stem plants might need 3-4 BPS. Small tanks under 20 gallons can often get away with less than 1 BPS.
If you're shopping for a complete CO2 setup, our guide to the best CO2 system for aquarium covers regulators, cylinders, and diffusion options together. For a direct comparison of reactor models, the best CO2 reactor roundup breaks down inline and in-tank options by tank size.
Cleaning and Maintenance
An in-tank reactor needs cleaning every 4-6 weeks. The impeller gets coated in biofilm, and the chamber walls develop algae. Both reduce efficiency.
The cleaning process is straightforward. Remove the reactor from the tank, disconnect the CO2 line, disassemble the chamber, and rinse all parts with warm water. Use a small brush to clean the impeller and any narrow passages. Avoid soap or harsh cleaners since residue can kill beneficial bacteria.
Some hobbyists soak the parts in diluted white vinegar (10:1 water to vinegar) for 20 minutes to break down calcium deposits. Rinse thoroughly before reassembling.
If you notice CO2 efficiency dropping between cleanings, the culprit is almost always biofilm on the impeller reducing flow rate through the chamber.
Common Problems and Fixes
CO2 Bubbles Escaping the Tank
If you see a stream of fine bubbles leaving the reactor outlet, the reactor is undersized for your flow rate or the CO2 input is too high. Try reducing the bubble rate by 25% and check whether the gas pocket inside the chamber is too large. Purge the gas pocket and see if efficiency improves.
Plants Still Not Growing Well
If your drop checker is green but plants are still showing deficiency signs, CO2 may not be distributing evenly. Reposition the reactor outlet toward your main circulation current. Check that your turnover rate is adequate. A planted tank generally needs 5-10x turnover per hour for good CO2 distribution.
Water Gets Sucked Into CO2 Line
This is called backflow and it happens when CO2 pressure drops below water column pressure. Install a check valve between your regulator and the reactor. Check valves cost a few dollars and prevent water from reaching your regulator, which can cause corrosion damage.
FAQ
Can I use an in-tank reactor with a canister filter? Yes, and it works well. However, if you have a canister filter, you might prefer an inline reactor instead, which stays completely outside the tank. In-tank reactors are most useful when you don't have an external filter available for inline connection.
Does an in-tank CO2 reactor affect water pH? Yes. CO2 dissolves into carbonic acid, which lowers pH. This is expected and desirable in planted tanks, where a pH of 6.5-7.0 with adequate CO2 is common. The drop is gradual during the CO2 injection period (typically 8-10 hours per day) and recovers overnight. A large, sudden pH crash usually means too much CO2.
How do I know what size reactor to buy for my tank? Match the reactor's rated flow range to your pump's output. A reactor rated for up to 60 gallons typically needs 50-150 GPH through it. Check the manufacturer's GPH spec, not just the gallon rating. Undersized pumps mean the CO2 accumulates and escapes; oversized pumps mean the water passes through too fast for full dissolution.
Do in-tank CO2 reactors make noise? Some do, some don't. Passive models that attach to a filter outlet are completely silent. Models with their own small pump can produce a faint hum. If noise is a concern, look for passive designs and pair them with a quiet pump.
Key Takeaways
An in-tank CO2 reactor gives you near-complete CO2 dissolution without requiring an external canister filter. It's a practical middle ground between the efficiency of an inline reactor and the accessibility of a simple diffuser. Size it correctly to your tank volume and pump flow rate, keep it clean, and rely on a drop checker rather than guessing at your CO2 levels. That combination will get your plants the CO2 they need without wasting gas or stressing your fish.