Nutrients and Water
Leave the mixing of hydroponic nutrients to the chemistry wizards. Many ready-made nutrient solutions that contain all of the necessary macronutrients and micronutrients that most plants will need. These solutions have been specially created for hydroponics. Don't haphazardly use fertilizers developed for use on soil-based plants. Most of these products contain residues that could clog hydroponic systems.
Also, the concept is different (and the creators of these different nutrients are well aware of that fact). Soil-based fertilizers are meant to supplement the minerals and matter in your soil. In hydroponics, the nutrient solution is all your plants can rely on for external nutrition (for information on how plants create some of their own food, see photosynthesis). In many ways, this simplification of variables works in your favor in hydroponics.
Hydroponic supply stores have a variety of nutrient mixes for specific crops and growth cycles. Always store mixed solutions out of direct sunlight to prevent any algae growth. For related discussions involving nutrient solution, see conductivity, macronutrients, micronutrients, mineral deficiency, and pH.
Any gardener needs to be confident about their water quality. This is especially true for hydroponic gardeners. The water you use in your hydroponic system needs to be pure. It is always a good idea to test your water source before adding nutrients so you aren't adding an element that is already present. In small systems, it would be wise to use distilled water.
If you are starting a larger hydroponic operation, it would be a good idea to have a water analysis completed. Factors such as sodium chloride (NaCl, or salt) content and hardness will be of great use to growers. Also, groundwater can have elements normally not present in conditioned water. A key piece of advice: Get to know your water!
Further Reading