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CO₂

Drop checker

The reason a drop checker is better than a pH/KH calculation is that it uses a reference water of known composition, so tannins and other buffers in the tank cannot skew it. It lags by an hour or two, which makes it a trend indicator rather than an alarm.

Short answer

A small glass vessel holding a 4 °dKH reference solution with pH indicator. Its colour reports the CO₂ concentration that has equilibrated into that known water.

What it does

  • Indicates dissolved CO₂ via colour change in a reference solution

Strengths and where it fits

Strengths

  • Immune to the buffering assumptions that break the pH/KH estimate
  • Cheap and passive

Good fit for

  • Any tank with CO₂ injection

Limitations and where it is the wrong choice

Limitations

  • Lags by one to two hours
  • Must use a 4 °dKH reference solution, not tank water
  • Indicator solution degrades and needs replacing

Poor fit for

  • Tanks without injection, where it tells you nothing useful