LIGHTING & CO₂

Drop Checker

Bromothymol-blue indicator that reads dissolved CO₂ from a standing colour change.

Planting stage Lighting

Unplug before servicingHands out of the water firstDrip loop on every cableTest before you dose30 ppm CO₂ ceilingCycle before stocking

Lighting

Specification
Reference solution	4 dKH
Indicator	Bromothymol blue
Response time	1–2 hours
Colour scale	Blue → green → yellow
Position	Opposite end from diffuser
Solution life	2–4 weeks per fill

Technical parameters
Blue	< 15 ppm CO₂
Green	~30 ppm CO₂ — target
Yellow	> 40 ppm — reduce injection
Reading time	Take at mid-photoperiod
Refill interval	Every 2–4 weeks
Placement	Furthest point from diffuser

A drop checker is a small glass bulb holding a few drops of 4 dKH reference solution with bromothymol blue indicator, separated from the tank water by an air gap. CO₂ diffuses across that gap until the solution reaches equilibrium with the tank, which takes one to two hours.

The colour then tells you the CO₂ concentration directly, because the carbonate hardness of the reference solution is known: blue means under about 15 ppm, green means roughly 30 ppm, yellow means above 40 ppm and is dangerous to livestock. Using a fixed 4 dKH reference rather than tank water is what makes the reading meaningful — it removes tank KH from the calculation.

This is why a drop checker outranks a bubble counter. The bubble counter measures what you are putting in; the drop checker measures what has actually dissolved, after losses from surface agitation, plant uptake and degassing. It is slow and it is coarse, but it is the honest number.

Safety and welfare

Yellow at any point in the photoperiod means livestock are at risk. Reduce the bubble rate immediately and increase surface agitation.
Place the checker where you can see it without opening the tank. A reading you have to hunt for is a reading you will stop taking.
The indicator solution is not harmful in the quantities used, but keep it out of reach of children and pets.

Using it well

Read the colour at mid-photoperiod, not first thing in the morning. Before lights on the reading lags reality by hours.

Put the checker at the far end of the tank from the diffuser. That is where CO₂ is lowest, so it is the worst case you are managing.

If the reading never goes green no matter how high you push the bubble rate, the problem is flow, not gas. The CO₂ is not being distributed.

Pairs with

CO₂ regulatorCO₂ diffuserBubble counterCanister filter (for flow)

Worth knowingThe only instrument that measures dissolved CO₂ rather than injected CO₂.

Note. Parameters are conventional ranges across the category, not the claim of any one manufacturer. Nothing on this page is for sale and no outcome for any plant or animal is guaranteed. Simulator output elsewhere on this site is a simplified model, not a measurement — confirm everything with a test kit and the equipment's own instructions.

Drop Checker

Using it well

Pairs with

Works alongside