A CO2 drop checker is a small glass of indicator solution that sits in your tank and shows CO2 by colour: blue is low, green is the target and yellow is high. For most injected tanks, the best buy is a plain glass kit with ready-mixed 4 dKH solution. Mount it at mid-height or lower, allow about two hours, and treat green as a minimum, not as an exact 30 ppm.
If you're choosing a whole system first, start with the aquarium CO2 system guide. This page covers only the checker.
How to read this page
These picks are types of drop checker, not products I've tested side by side. I've never had a drop checker or any other CO2 test in my tanks, and I haven't run pressurised or DIY CO2 on any of them, so everything here comes from the sources cited. The models named are examples with published specifications. Prices are what each retailer showed on 3 November 2026, and they'll change.
The short answer
- Most CO2 tanks: a glass kit with ready-mixed 4 dKH solution — pick 1.
- A rimless tank, or you want the front glass clear: a hang-on checker — pick 2.
- A nano tank: a compact checker — pick 3.
- You'd rather match against a printed scale: a long-term test — pick 4.
- You already own a checker: just the 4 dKH solution — pick 5.
Whichever you buy, it's a rough gauge. The 2Hr Aquarist says it can show that some minimum level of CO2 is present, but it isn't a reliable way to set a level.
The picks
Quick comparison
| # | Type | Best for | Where it sits | Example | Price seen |
|---|---|---|---|---|---|
| 1 | Glass kit with 4 dKH solution | Most CO2 tanks | Inside, on a suction cup | CO2Art kit, 3 × 5.5 cm, with 10 ml solution | £15.99 (Aquarium Gardens) |
| 2 | Hang-on checker | Rimless tanks | Over the rim | UNS Drop Checker HD, about 70 mm tall, with 15 ml solution | $13.99 (Buce Plant) |
| 3 | Nano checker | Small tanks | Inside, on a suction cup | Aquasabi Nano, 40 × 40 mm, checker only | From €6.90 (Aquasabi, range price) |
| 4 | Long-term test with colour scale | Matching a printed scale | Inside the tank | JBL ProAquaTest Permanent CO2/pH, complete set | €12.90 (Aquasabi) |
| 5 | 4 dKH solution | Refills | In your checker | CO2Art 15 ml, dye included | €6.99 (CO2Art) |
How I chose
Four things separate one drop checker from another:
What goes in it. A drop checker is a pH test sealed off from the tank by an air gap, so only CO2 reaches the liquid. CO2Art says that's why tank water is a poor fill: other acids and bases affect its pH. A solution set to 4 dKH avoids that, so kits with one included ranked first.
How easy the colour is to read. CO2Art says shape doesn't change performance, but some shapes make the colour harder to see. That's a real issue, because The 2Hr Aquarist names colour interpretation as the main weakness of these devices.
Where it mounts. Most fix to the glass with a suction cup. Hang-on models avoid that, but they sit higher in the water.
What it costs to keep going. The glass is a one-off purchase. The solution is the running cost, and it needs replacing about monthly.
What green really means
This is where the sources split, and it's the most useful thing to know before you buy.
- Aquarium Science: green at 4 dKH is 25–35 ppm, with lime green at 30 ppm. It calls drop checkers dependable for holding 30 ppm or lower.
- CO2Art and Green Leaf Aquariums: about 30 ppm. Green Leaf adds that this applies only with a 4 dKH solution, and that a checker isn't an exact meter.
- Aquasabi and Green Aqua: a target of about 20–30 mg/l, which is the same as ppm.
- The 2Hr Aquarist: a true 4 dKH solution with bromothymol blue turns green at 10.7 ppm. It calls the advertised 30 ppm "quite far off" and recommends the 1 pH drop method or a CO2 test kit instead.
I can't settle that from here, because I've no CO2 tank to test it on. The cautious reading is to use green as a floor. If the checker is blue, you're short. If it's green, some CO2 is getting through, and even that can be useful: The 2Hr Aquarist says 10–15 ppm will grow most easier plants and common carpets.
For demanding plants, don't rely on the colour. Use the pH-drop method as well. The 2Hr Aquarist also tested the Hanna HI3818 kit and found it accurate to about ±5 ppm against an analyser.
Each pick, honestly
1. Glass drop checker kit with 4 dKH solution — best overall. Everything you need comes in one box. Aquarium Gardens lists CO2Art's kit with a glass checker, 10 ml of solution and a suction cup, with no mixing. Buce Plant sells the UNS SD with 15 ml of fluid for $13.99, and Green Leaf Aquariums' Glass Orb kit comes with 2 oz of 4 dKH solution for $32.99. Drawback: Aquarium Gardens says its 10 ml bottle lasts around six months, so you'll be buying more.
2. Hang-on drop checker — best for rimless tanks. The UNS HD hangs on the rim, and Buce Plant says it fits rims up to 10 mm thick. The Cal Aqua Labs Clip Nano does the same, for rimless tanks with glass under 10 mm thick. Drawback: depth. Aquasabi gives the UNS HD an immersion depth of about 45 mm, and The 2Hr Aquarist says a checker near the surface is in the worst place, because it can catch CO2 bubbles.
3. Nano drop checker — best for small tanks. Aquasabi's Nano is 40 mm high and 40 mm wide, against 55 mm by 50 mm for its standard version. In a small tank, that's less glass in view. Drawback: there's less colour to judge, and CO2Art warns that some shapes are harder to read. Aquasabi sells the reagent separately, so add a bottle to your order.
4. Long-term CO2 test with colour scale — best for reading against a printed scale. These replace guesswork about the shade with a chart. Dennerle's test uses a six-colour scale in mg/l and runs its indicator undiluted, which it says keeps out the acids that distort other tests. Aquasabi's JBL listing says its colour scale covers CO2 and a pH range of 6.4–7.8. Drawback: a printed scale doesn't remove the problem The 2Hr Aquarist describes, because you're still matching close colours by eye. Dennerle lists its test at €22.99, which is more than a basic glass checker.
5. 4 dKH drop checker solution — best for refills. CO2Art's 15 ml bottle comes ready-made with the indicator in it. Green Leaf Aquariums' 118 ml bottle costs $12.99 and works out far cheaper per ml, but it has no indicator, so you add your own. Drawback: you have to check what's in the bottle. Green Leaf's FAQ says some premixed solutions include both the 4 dKH water and the dye, and its own doesn't.
Free tool
Starting bubble rates are quoted per litre or gallon, so work out your tank's real volume before you start adjusting towards green.
Aquarium volume calculator →How to set one up
- Rinse the checker, as Green Leaf Aquariums advises for its kit.
- Fill it upside down. Aquarium Gardens says to fill the reservoir halfway, and CO2Art's solution page says about two-thirds. Follow your pack, and leave the air gap.
- Mount it fully under water, at mid-height or lower. Green Aqua says to keep it out of direct line with the diffuser and somewhere with good flow.
- Wait about two hours, then read it. Do the same after every change.
- Judge it late in the CO2 period. Green Leaf Aquariums says a yellow reading in the morning may reflect the previous injection period.
- Clean it and replace the solution about every month.
Placement advice varies. Aquarium Gardens says about 15 cm below the surface, and The 2Hr Aquarist says the mid or substrate zone. On a shallow tank those may be the same spot; on a tall one, go lower. Aquarium Science suggests moving it around to find out what each area gets, and using more than one checker on a large tank.
Why the colour lags
The liquid doesn't touch the tank water, so CO2 has to cross the air gap first. The figures vary:
| Source | How long it takes |
|---|---|
| Buce Plant (UNS) | At least 1 hour |
| Aquarium Gardens | Up to 2 hours; about 1–2 hours behind the tank |
| The 2Hr Aquarist | 90 minutes or more for a stable colour |
| ADA (via Garnelio), Green Leaf Aquariums' Orb kit | 2–3 hours |
| Aquarium Science | Several hours; roughly a three-hour average |
| CO2Art | May take 4–5 hours |
Whichever is right for your checker, the lesson is the same: make one small change, then wait. CO2Art warns that reacting too quickly creates a yo-yo effect that can stress fish and plants and encourage algae. The lag can also lead you to add more CO2 than you need, says Aquarium Science.
Your fish beat the colour
Green doesn't guarantee safety. Green Leaf Aquariums says that if fish or shrimp gasp or breathe abnormally, you should stop CO2 injection straight away and add aeration, without waiting for the colour to change. See fish gasping at the surface.
What you can skip
- Expensive glass for better accuracy. CO2Art says shape doesn't affect performance. Aquasabi lists the ADA Drop Checker at €49.90, and its own checker from €6.90. Buy ADA for the look, not the reading.
- Filling it with tank water. ADA's instructions do this, with a reagent. Garnelio's listing warns that very acidic water turns the liquid yellow straight away, and CO2Art says tank-water readings are unreliable.
- Mixing your own 4 dKH. Aquarium Science gives baking-soda recipes, but they need distilled water, a settling time and a retest. A bottle costs a few pounds.
- A drop checker in a tank with no injected CO2. Aquarium Science says uninjected water sits at about 2–3 ppm. A low-tech planted tank has nothing here to tune.
If it won't turn green
Don't simply add gas. Green Leaf Aquariums says a stuck blue can come from bad indicator preparation, poor placement or too little waiting. Aquarium Gardens says that if you can't get a reading, the CO2 isn't dissolving well, so improve circulation or aim the diffuser into the flow. If bubbles are escaping to the surface, compare CO2 diffusers next.
The best check is still the plants. The aquarium CO2 system guide covers what better new growth looks like, along with the full pH-drop method.