Nutrients for hydroponic systems
Rain water and municipal water already has enough minerals and nutrients for plants to grow but not to thrive. If you want plants to really thrive and increase your yield, you will need to add specific nutrients for specific plants. Adding nutrient minerals and adjusting pH can help your plants grow larger, faster and in some cases more fruit. As you grow more plants from the same water tank or reservoir, they will take nutrients out of the water at a faster rate which means you will need to replenish the nutrients when you add more water. While I have personally grown many plants on tap water alone, the difference when using a nutrient solution is much faster and far superior growth.
When adding nutrient solutions, there are many different types and you need to pay attention to depending on what you are growing. Macro nutrients is the most important aka NPK or Nitrogen, Phosphorus and Potassium. NPK is not negotiable nutrients for plants and in any quantity will ensure good growth. However, to thrive, the ratio % between each 3 elements (or technically speaking the chemical compounds of these elements) is critical for every different species of plant. Typically, high nitrogen ratio is for growth and a high ratio of phosphorus is for flowering. The USDA has very handily pre-calculated the exact NPK nutrient ratios for farmers on their website here.
One of the advantages to growing hydroponically is being able to near fully automate plant growth while you are away from home. Homefarmhydro experimented with a 12 week period of automated tap water replenishment. The results were clear, plants were alive but not thriving. Leaves were discolored and weak but still saw tremendous growth. In hindsight, I probably didn't test with the best plant types but the results were conclusive in that hydroponics can be maintenance free. Greenthumbs no longer need to have a friend or relative keep your plants alive while you are on a long vacation.
I also have my own natural nutrients solution that I make from left over food waste. This is part of another experiment to reuse and recycle. To participate in a circular economy, we need to learn how to reuse from sustainable resources. I have a banana with breakfast, drink coffee every morning and tea every afternoon. Just from these daily ingredients, I drying out the waste, grind it up and store it. These leftovers will keep for a long time in a air tight jar and can be reused in a nutrient solution to further facilitate the NPK requirements needed for strong plant growth.