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Send your water to a lab to be tested, if you can. With good, "soft" water, you'll be able to add any nutrients your plants need for their optimal growing season. With "hard" water, you may need to use reverse osmosis methods to filter out any unwanted heavy metals present in your water.  You can also use a dissolved solids meter to check your water regularly. This is also called an electrical conductivity (EC) or parts per million (PPM) meter. Calcium and magnesium carbonates are quite common ingredients in both tap water and well water. Each are necessary nutrients for plant growth, but in limited amounts. Knowing how much of these elements are present in your water determines how much, if any, you need to add. The essential nutrients used include calcium nitrate, potassium sulphate, potassium nitrate, mono potassium phosphate, and magnesium sulphate. Each element involved in these nutrients provides a different benefit.  Hydrogen forms water by combining with the oxygen. Nitrogen and sulfur are essential to the supply of amino acids and proteins. Phosphorus is used in photosynthesis and overall growth. Potassium and magnesium act as catalysts in the creation of starches and sugars. Magnesium and nitrogen also play a role in the production of chlorophyll. Calcium is a part of the make-up of cell walls, and plays a role in the growth of cells. Micronutrients, also called trace elements, are also essential, but they are only required in very small amounts. These elements effect growth, reproduction, and the effect that other nutrients have on the plant.  Micronutrients used include boron, chlorine, copper, iron, manganese, sodium, zinc, molybdenum, nickel, cobalt, and silicon. There should be at least 10 trace elements present in your nutrient mix. The best temperature for plants is tepid: neither warm nor cold to the touch. If your solution is too cold, your plants won't germinate. They may mold or rot. If your solution is too hot, your plants might die from stress or oxygen deficiency. The optimal temperature for water is between 65 degrees (18 C) and 80 degrees (27 C).  Plants grown in colder climates will thrive in cooler water, while plants grown in warmer regions prefer warmer water. When you add new water to your reservoir, make sure it's approximately the same temperature as the existing reservoir water. You can use a pH meter to check your balance. You want your pH balance to be between 5.5 and 7.0. Your water's pH balance ultimately impacts the plants' ability to take in nutrients.  It's normal for pH balances to wander up and down. The balance will naturally change as elements are absorbed by the plants. Avoid adding too many chemicals as a reaction to the varying pH balance. If you have poor quality growing medium, this may affect the stability of your pH balance. Most municipal water systems raise the pH level of their water by adding calcium carbonate. The average pH balance of city water is often as high as 8.0. Remember that pH measuring kits will show different levels in different temperatures. Check your water temperature before adding chemicals to your water.

Summary:
Know what's in your water. Familiarize yourself with essential macronutrients. Choose the right micronutrients. Check your water temperature. Keep the right pH balance.