Different dissolved salts are taken up less effectively as pH varies from low (acid) to high (alkali) A good point to aim for is between 5.8-6.8, if pH rises much above 7.5 then problems can occur.The graph plots the total nutrient available at increasing pH values. It peaks at around 6.5, both side of which the total available drops.
A plant's ability to take up nutrients is determined by pH and temperature. If your soil is too acid or too alcaline then the plant will struggle to extract what it needs. This in turn leads to deficiency symptoms
Maintain the nutrient solution at about 24-27 deg C.
Nutrient manufacturers publish the preferred concentrations and EC readings for their products on the packaging. Usually 1.5-2.5 millisiemens or 15-25 CF is the target, less than this will result in deficiencies, more than 30 CF and the plant will also deteriorate
A word of caution though, it is easy to over fertilize and burn the roots, especially with younger plants.
Even if you aim to grow a single type of plant hydroponically you usually end up growing two, your favorite crop and algae. Algae are an unwelcome addition as it uses nutrient, can clog pipes and pumps, can harbor pests and looks a mess.
Algae are one of the oldest waterborne plants on the planet and it has become supremely well adapted to taking any advantage it can to grow. Hydroponic systems are perfect for algae, lots of liquid nutrient, oxygen, warmth and light. Limiting light is the best way to prevent its growth. Black plastic components should be used where possible, if not then light should be excluded by covering. Dustbin liners are good for this and when cardboard or polystyrene sheet is placed inside it make an adaptable material for boxing in nutrient sumps or covering exposed substrate.
Many systems include an air pump and air stone to aerate the nutrient solution. This action is used in simple aeroponics systems to provide a fine spray as the bubbles burst at the surface. In all other systems it's used to provide circulation of the nutrient within the holding container, helping to keep it fresh.
Oxygen is poorly adsorbed by water, the larger the surface area between the two the better. As the bubbles add very little to the total area, they add very little oxygen directly. Far greater is that added at the surface of the solution where the area is large. Bubbles help by causing a gentle flow in the solution, as they rise the surrounding liquid also rises. This flow moves the lower part of the solution to the surface where it can take up oxygen. This movement can also be used directly in some pot-based systems to move nutrient up onto the substrate through a tube, replacing the pump.
In continuous flow systems there is often no need for anything other than the returning nutrient to increase surface flow. In other systems it's possible to divert some of the flow from the pump to cause currents in the solution by splitting the outlet into two pipes using a T-piece and a valve to balance the output