Potassium carbonate (K2CO3) is a white salt (supplied as a powder) which is soluble in water.
This chemical is used in planted aquariums to increase the stability of the water pH by increasing the KH of the water and adding potassium to the water which plants use as a major nutrient.
It is often preferred over bicarbonate of soda, as it contains no hydrogen ions. However if there is any acids present in the water, then the K2CO3 will react to form Potassium bicarbonate (KHCO3) which produces extra H+ ions and so the pH will rise.
- Potassium carbonate (K2CO3) contains 56.58% potassium (K).
When added to water, this powder turns the water milky for a short time as it dissolves.
Adding 80g of Potassium carbonate to 1 litre of water will give a stock solution of 5.0% K2O (4.2% Potassium) W/W
- Adding Potassium carbonate at the rate of 0.011g per litre to a tank will increase the potassium level by 6 ppm 
- ↑ Fluidsensor online - Using chemical fertilisers in planted aquariums - Potassium carbonate