Apart from nitrogen and phosphorus, potassium plays an important role in ensuring the proper growth of plants. While nitrogen and phosphorus concentrations in fish tanks stay usually at adequate or even elevated levels, keeping desirable potassium level requires its supplementation and regular control. Plants deficient in potassium become yellow and holes appear in their leaves over time. For majority of plants, an adequate concentration of potassium in water is 10 – 20 ppm, and its depletion is largely dependent on quantities and growth rates of plants. Although a high level of potassium in water is not toxic, it may contribute to impaired uptake of other important elements. It is therefore required to use potassium supplements with care.