Easy-Life Nitro and Easy-Life Fosfo are used to compensate for a deficiency of nitrogen (N) and phosphorus (P), respectively. A deficiency of these macronutrients immediately leads to faltering growth or even to growth arrest.
A shortage of N and P can quickly arise in plant aquaria that contain very few fish or no fish at all, because very little or no fish feed is used. Together with fish excrement, fish feed is the most important source of N and P. A deficiency can also arise in aquaria with a very high density of plants. Nitro (which contains nitrate NO3) and Fosfo (which contains phosphate PO4) immediately compensate for these deficiencies.
The Redfield ratio is usually used to decide on the correct ratio between nitrate (NO3) and phosphate (PO4). See further on for a detailed explanation.