Potassium is a very important nutrient. Potassium (K), nitrogen (N) and phosphorus (P) – NPK are the most important nutrients for water plants, because they are used in a huge amounts so it’s important to supply them artificially. Potassium is for aquatic plants accessible in form of potassium cation K+.
Potassium deficiency is often a huge problem. There is usually not enough of this macronutrient in water and we have to supply it through fertilizer. Level of potassium in nature is from 2 ppm to 100 ppm. Unfortunately, our water doesn’t contain more than 2 ppm. To increase this concentration we should use Potassium Sulphate (K2SO4). Ideal potassium level in your tank should be from 10 ppm to 50 ppm. You want to add weekly 20 ppm of potassium to your high light aquarium or 10 ppm for low tech tank without CO2 supply.
Some of the potassium is contained in fertilizers like Potassium Nitrate (KNO3) and Monopotassium Phosphate (KH2PO4) but it’s a smart idea to add some extra potassium from Potassium Sulphate (K2SO4).
If you are using as a fertilizer KNO3, you should add the same amount of Potassium Sulphate (K2SO4). Example:
If you are using for your 25 gallon tank 1/2 tsp of KNO3 weekly, you should use also 1/2 tsp of K2SO4 weekly. If you’re not using KNO3 because you have enough of nitrogen (N) in your tank, you can just use 1 tsp of K2SO4 instead.