Dependencies between aquarium water parameters

Theoretically acidic water can be hard and alkaline water can be soft. But it's a "laboratory" case. Mostly in nature acidic water is soft and alkaline water is hard. Aquarium of "tropical rainforest" kind usually has soft acidic water with some organic matters, such as tannins and humic acids. Tank for Malavi / Tanganyika Cichlids, should be filled with more hard alkaline water. Water of a goldfish tank should be adout neutral pH and medium hardness.

Figure 1 shows a typical statistic of dependence betwee water hardness and pH. Aquahobbist should also take in account the fact, that water parameters are not stable in time. For example, in a rainforest dGH gets softer in a rain season; in a daytime illuminated plants consme CO₂ it caused that the water gets more alkaline.

Season changes of water parameters influence fish biology in a habitat, so corresponding changes can be a spawning stimulus in a tank. But such chanes should be done gradually.