Planted Tank (CO2)

In a high-tech planted tank, water chemistry isn't just about keeping fish alive. It's a direct input to how well your plants grow. Calcium and magnesium are macronutrients: plants use calcium for cell wall structure and magnesium as the central atom in chlorophyll (the molecule that drives photosynthesis). A GH below 4 °dH and you'll see twisted new growth (calcium deficiency) or yellowing leaves with dark veins (magnesium deficiency, also called chlorosis).

KH takes on a different significance in CO2-injected tanks. When CO2 dissolves in water it forms carbonic acid, which lowers pH. KH (bicarbonate) resists that pH drop. The practical consequence: higher KH means you need to inject more CO2 to reach the same target concentration, and you get wider pH swings between lights-on (CO2 running) and lights-off. A KH of 3–6 °dH is the workable range. Below 3, pH becomes dangerously unstable. Above 6, you're fighting the buffer with every bubble.

Here's the counterintuitive part: "soft-water plants" are actually low-KH plants, not low-GH plants. A tank with GH 10 and KH 1 will grow delicate species like Rotala and Tonina beautifully. A tank with GH 1 and KH 10 won't grow them at all. Understanding this distinction is the single most useful thing you can learn about planted tank water chemistry.