Plants need fertilizer for them to flourish. In traditional farming or gardening, plants absorb nutrients from the soil and additives like chemical fertilizer, manure, and compost. In terms of hydroponic farming and gardening, plants aren't grown in soil, so the nutrients they need must be supplied directly through the hydroponic solution they're watered with. These nutrients are categorized into 2 groups: micronutrients and macronutrients. Macronutrients are nutrients that plants need in large amounts, including hydrogen, oxygen, nitrogen, sulfur, phosphorous, carbon, magnesium, potassium, and calcium. Micronutrients are needed in smaller amounts, including nickel, chlorine, molybdenum, boron, iron, copper, zinc, and manganese.
When these essential elements aren't present, plant wouldn't be able to build important molecules, complete a life cycle, and go through enzymatic reactions. For a hydroponic gardener or grower, this means that when hydroponic nutrients aren't supplied, plants won't be able to bear fruits but if they do, whatever they make would be poor quality.
PH levels of hydroponic nutrient solutions is another important element to consider. It has a big effect on how much nutrition plants can get. It's very important to check the pH level regularly, ideally daily regardless if you are very careful at measuring and mixing the nutrient solutions. Different plants have a bit different needs in terms of the nutrient concentration and pH value. If you are thinking of growing a vast variety of plants, see to it that you know the particular requirements for each type so you could group them based on the things they need.
Also, a certain need of a plant change under different conditions, such as the season, temperature, and weather. This isn't a problem with indoor hydroponic gardens with controlled environment.
The hydroponic nutrient solutions should be kept at a stable temperature. The ideal is room temperature, between 70 to 78 degrees Fahrenheit. This is more of an issue for outdoor hydroponic systems. During the cold months, use portable water heaters that go inside reservoir to keep the nutrient solution warm. During summer time, keeping the water reservoir in a shaded area and adding cool water every now and then is sometimes enough to maintain the right temperature.
You could get pre-made hydroponic nutrient solution, or you could make your own nutrient solution. Small-scale farmers and gardeners and those who make hydroponics a hobby, use pre-mixed powder or liquid concentrates that are mixed in water. Large-scale hydroponic growers generally prepare their own solution to plants' specific needs, using bulk concentrates.