CO₂ in a planted aquarium: setup, drop checker and dialing it in

Do you need CO₂?

Not always. A tank with easy plants and low to moderate light does fine without injected CO₂. Injection speeds up growth and pays off mainly if you want strong reds or a dense carpet of small foreground plants, which usually struggle without it.

The faster growth comes with more work: plants use more nutrients, so fertilizing has to keep up, you trim more often, and light, CO₂ and nutrients have to stay in balance. CO₂ on its own will not fix algae caused by too much light or too few nutrients, and it needs consistency – the same schedule and the same rate every day.

What a pressurized setup includes

• a cylinder – from a 60 L SodaStream cylinder (425 g in Europe, 14.5 oz in the US) with an adapter to larger refillable cylinders that last much longer; the adapter and regulator must match the cylinder thread. Adapters screw onto the threaded (blue) cylinders; the newer pink Quick Connect cylinders clip in rather than screw in, so check that the adapter fits yours before you buy,
• a regulator with a solenoid – the solenoid is an electric valve a timer opens and closes,
• a needle valve for fine adjustment of the flow,
• a bubble counter to show that gas is flowing and that the rate stays the same,
• a check valve that keeps tank water from flowing back into the regulator,
• a diffuser or inline reactor to dissolve the gas,
• a drop checker to show how much CO₂ is actually in the water.

A dual-stage regulator keeps the working pressure, and so the flow, steady until the cylinder is empty. With a single-stage regulator a nearly empty cylinder can dump the rest of its gas at once – see troubleshooting below.

How long a cylinder lasts depends on your rate and how many hours a day CO₂ runs. The first cylinder tells you – which is why you log each swap.

Reading a drop checker and checking the pH drop

The usual target is 20–30 ppm. Two methods check each other:

• a drop checker with 4 dKH reference solution: blue – too little CO₂; green – roughly 20–30 ppm, with lime green at the top of that range (about 30 ppm); yellow – too much, check your fish. With shrimp or sensitive fish, stay at a solid green,
• the pH drop: take a sample of tank water in the morning before the CO₂ comes on and aerate it with an air stone (or shake it repeatedly) until two readings in a row match. Just letting it stand takes a day or two and may not be enough. Then measure the tank pH late in the CO₂ period, just before it switches off. A drop of 0.8 corresponds to roughly 20 ppm, a drop of 1.0 to roughly 30 ppm.

The pH drop should agree with the drop checker and with how your fish behave. If they disagree, do not add more CO₂ until you know why.

Fill the drop checker with reference solution, not tank water: the tank's own hardness and acids throw the reading off. Place it at the opposite end from the diffuser, out of the bubble stream, where you can see it easily. It reacts with a lag of about two hours, sometimes more, so it shows where the level was, not where it is now – wait before you turn the valve again.

KH and pH charts are less reliable. Aquasoil, driftwood and peat change pH on their own, so the chart can be far off.

Dialing it in and timing

• start with a low rate and raise it in small steps, waiting at least a day after each change,
• after every increase watch fish and shrimp, especially late in the CO₂ period,
• put the diffuser where the filter flow carries the fine mist around the whole tank,
• switch CO₂ on one to two hours before the lights so it is on target when they come on, and off about an hour before lights-out – the CO₂ left in the water covers the rest of the day,
• CO₂ stays off overnight – plants do not use it in the dark, and they respire.

Bubbles per second cannot be compared between tanks. Bubble size differs from one counter to another, and how much gas dissolves depends on the diffuser and the flow. Set the level by the drop checker and the pH drop; use the counter only to see that the rate has not changed.

In a shrimp tank raise CO₂ more slowly and stay near the bottom of the CO₂ range, around 20 ppm (a solid green drop checker). SUISO's pH target for that style is 6.5–7.8; with low KH, more CO₂ takes you below it. What shrimp handle worst is sudden change.

Troubleshooting

• fish gasping at the surface: turn the CO₂ off now and agitate the surface (raise the filter outlet, add an air stone); if it does not improve quickly, do a large water change with conditioned, temperature-matched water. If fish lie on the bottom or shrimp are affected, do the water change straight away. Then restart at a lower rate,
• black brush algae on leaves: fluctuating CO₂ is one of its main triggers; check that CO₂ starts at the same time every day and that the drop checker shows the same color day to day,
• a drop checker that suddenly turns bright yellow with no change on your side, and a bubble counter running much faster than you set it: with a single-stage regulator this is often an end-of-tank dump; close the cylinder and aerate the tank.

The cylinder gauge reads nearly the same pressure until the cylinder is almost empty, so it does not tell you how much gas is left. Once that pressure starts falling, swap the cylinder instead of running it dry.

Liquid carbon and DIY CO₂

Glutaraldehyde-based liquid carbon is sold as a carbon source for plants. How much of it plants actually use is disputed; what is clear is that it does not replace injected gas. It suits lower-light tanks or works as a supplement. Dose exactly to the label and never above it. Large doses stress more sensitive fish and plants, and Vallisneria leaves can melt. Seachem does not market Flourish Excel as an algaecide and advises against spot-dosing it, although hobbyists report that black brush algae recedes – which still leaves the cause, unstable CO₂, unfixed. This guide gives no dose: it differs by product, and sources disagree on where the limit for shrimp lies.

Yeast-and-sugar CO₂ and citric acid generators are a cheap way to start. Their output is less stable than a regulated cylinder and they need frequent refilling. Yeast CO₂ cannot be switched off at night: keep the surface moving or run an air pump on a timer overnight, and check the fish in the morning. For plants, steady CO₂ matters more than a high number.

What to log and why

Your CO₂ schedule, the drop checker color, the tank pH late in the CO₂ period, every change of the rate and each cylinder swap. When black brush algae or gasping fish appear, the log shows whether and when CO₂ changed. The intervals between cylinder swaps also tell you when the next one will run out – SUISO estimates the next swap once your journal has at least two, and you set the CO₂ schedule in the tank settings next to the lighting schedule.