KH test kits, compared

The CO2 calculator's whole output rests on one input most hobbyists measure the least carefully: KH. A pH probe is easy to trust because the number moves in front of you. KH is a count — drops, color changes, a strip you compare against a chart — and small measurement slop there gets amplified by the exponent in the formula. This page walks through the three common ways to test it, so the number you type in is actually worth trusting.

Titration drop kits

The standard method, and still the one most experienced hobbyists reach for. You add a reagent to a water sample drop by drop, counting until the color flips — each drop typically represents one degree of KH. Common hobby brands (API, Salifert, JBL, and others) all work on this same principle, just with different reagents and drop-to-degree ratios, so always check the specific kit's conversion rather than assuming "1 drop = 1 °KH" universally.

Accuracy: good, provided you count carefully. The most common error is misjudging the color transition — it's often a gradual shift rather than a sharp snap, and stopping half a drop early or late shows up as a real error in the final CO2 number.

Cost and lifespan: a bottle typically lasts many months of weekly testing for a single tank. Reagents do degrade over time, especially once opened, so treat a kit older than a year or two with more skepticism.

Test strips

Dip a strip, wait a few seconds, compare the color pad against a printed chart. Fast and cheap, but the honest tradeoff is resolution — most strips jump in increments of several degrees KH rather than single degrees, and reading a color chart under different lighting introduces its own error.

Where strips make sense: a quick sanity check between more careful titration tests, or monitoring a tank where you mainly want to catch a big swing rather than dial in an exact number. Not the best choice as your only method if you're actively tuning CO2 dosing.

Digital / electronic testers

A newer category — a handheld meter that measures KH electronically rather than by color comparison. These remove the human judgment call from reading a color transition, which is the single biggest source of titration error.

Tradeoffs: higher upfront cost than a drop kit, and like any electronic probe they need periodic calibration to stay accurate. For someone testing KH across several tanks regularly, the consistency can be worth it; for one tank tested weekly, a careful titration habit gets you most of the same accuracy for a lot less money.

Three habits that matter more than which kit you buy

• Test the tank water, not the tap. If you dose crushed coral, buffers, or remineralizers, your tank's KH can drift meaningfully away from your source water within weeks.
• Count drops slowly, one at a time, swirling between each. Rushing a titration is the most common reason two tests of the same water disagree.
• Don't trust a reagent bottle you can't remember buying. Old or improperly stored reagents drift, and a KH test that's quietly wrong is worse than no test at all, since it feeds a confidently wrong number into the CO2 formula.

Once you have a KH figure you trust, head back to the calculator and pair it with your pH reading to get an actual CO2 ppm estimate.