How to Use a CO2 Drop Checker

A drop checker is the simplest honest gauge of how much CO2 is actually dissolved in your tank. It works by lagging deliberately behind the water, so it reports a stable average instead of a jumpy live reading. Understanding what the colour means keeps your plants fed without gassing your fish.

How It Actually Works

The checker holds a pocket of reference liquid, a 4 dKH solution with a pH indicator dye, separated from the tank by an air gap. CO2 diffuses out of the water, across that gap, and into the reference liquid, changing its pH and therefore its colour. Because the gas has to cross the gap, the reading lags the tank by one to two hours, which is exactly the point: it smooths out the short-term swings.

Reading the Colour

Blue means too little CO2, green marks the sweet spot of around 30 ppm that most CO2 plants thrive on, and yellow means too much, the danger zone where fish start gasping at the surface. Aim for a colour that shifts from bluish-green in the morning to a solid grass green by mid-afternoon, and never let it tip over into yellow.

Placing and Maintaining It

Mount it on the glass opposite your diffuser and about two-thirds of the way down, out of the direct bubble stream, so it samples average tank water rather than a hotspot. Refresh the reference solution every couple of weeks, since it slowly picks up contaminants and its readings drift. Read it in the afternoon, a few hours into the photoperiod, once the CO2 has had time to equalise.

The Bottom Line

Green by mid-afternoon, never yellow, 4 dKH solution only, and refreshed every few weeks. Combined with watching your fish at the surface, a drop checker is the cheapest insurance against both starving your plants and suffocating your livestock.