Fluval CO2 Indicator Set

Fluval CO2 Indicator Set lets you see your planted aquarium's CO2 level at a glance. The glass drop checker holds a color-changing indicator fluid that shifts from blue to green to yellow as dissolved CO2 rises, so you can dial in the sweet spot for lush plant growth without guessing. It is the easy way to keep CO2 in the safe, effective range for your plants and fish.

Pair it with the Fluval Bio-CO2 Diffuser and Fluval Bio-CO2 Pro System to run and monitor CO2 together, and browse our live aquarium plants.

What's In the Set

• Glass raindrop drop checker with suction cup
• 10 mL (0.34 fl oz) CO2 Indicator Liquid
• Color check sticker for quick reference

Reading the Colors

How to Use

• Add 15 drops of the CO2 Indicator Liquid into the drop checker, then rotate it upright so the sphere is about one third full.
• Mount it with the suction cup roughly 10 cm (4 in) below the water surface, where you can see it easily.
• Allow about 12 hours for the fluid to react before reading the color, since it responds gradually.
• Refresh the indicator liquid about every 30 days to keep readings accurate.

Recommended Pairings

• Fluval Bio-CO2 Diffuser: the ceramic-disc diffuser that mists CO2 into your tank
• Fluval Bio-CO2 Pro System: the low-pressure CO2 source for planted setups
• Live aquarium plants: the plants that reward steady CO2 with faster, healthier growth

Frequently Asked Questions

What does it measure?

Dissolved CO2 in a freshwater planted aquarium, shown as a color you can read at a glance.

How long until the color settles?

Give it about 12 hours to react. Because of that delay, the color reflects your CO2 level over time rather than the last few minutes.

Do I need to refill it?

Yes. Replace the indicator liquid about every 30 days so readings stay accurate. Refill bottles are sold separately.

Freshwater or saltwater?

Freshwater planted tanks. CO2 monitoring is for aquariums that inject CO2 for their plants.