JBL ProAquaTest KH Carbonate Hardness measures carbonate hardness in fresh and marine aquariums. You add the reagent to a measured sample a drop at a time and count the drops until the colour turns from blue to yellow. The count is your KH, which makes it one of the easier tests to read with confidence.

Key Features

• Counts drops to a colour change: blue to yellow, and the number of drops is the reading. No shade to judge.
• Fresh and marine: JBL lists both, and KH matters in each for different reasons.
• JBL's testing schedule: weekly while a tank is being set up, monthly once it is established.
• The value that holds pH steady: carbonate hardness is the buffer. If it is low, pH will not stay put whatever else you do.
• Needed for CO2 calculations: pH and KH together are what give you a CO2 figure in a planted tank.
• Rinse the vials in tank water: a trace of the last reagent left in the glass is the most common reason a test reads oddly.

Why KH Is The One To Watch

• In a freshwater tank, KH under about 4 degrees leaves the pH free to swing between day and night, and the swing is what harms fish.
• In a marine tank, KH is consumed by corals and needs topping up. JBL's ocean reference is 10 to 12 degrees dKH.
• In a planted tank with CO2, the pH you read means nothing without the KH to go with it, because the two together give you dissolved CO2.
• Test KH before you adjust pH, never after. Adjusting pH in water with no buffer is how a tank crashes.

Getting A Reading You Can Trust

• Read the colour in daylight, against something white, and not through the side of the aquarium.
• Hold the reagent bottle vertical rather than tilted. A drop counted at an angle is not the same size as one counted straight down, and drop tests are counted in drops.
• Take the sample from mid-water, away from the filter outlet, and test it straight away.
• Rinse the vials in tank water before and after, and never with detergent.
• Write the numbers down with the date. One reading tells you where you are; a run of readings tells you which way the tank is going, which is the useful part.

Good To Know

• Reagents have an expiry date and they drift as they age. A test that has been open for years will quietly give you the wrong answer.
• Store the kit closed, cool, out of direct sun and away from children. The reagents are chemicals, not accessories.
• Heat and humidity are hard on reagents. Keep the case indoors rather than in a garage or on a balcony.
• JBL sells these tests both as a complete set and as a reagent refill. If you need the glass vials, syringe and comparator block as well as the reagents, check which one you are ordering.
• Count the drops out loud or mark them off. Losing count halfway is the usual reason a KH test gets repeated.