All hydroponic nutrient solutions must be changed after a short period since evaporation causes concentration of salts in the water. Adding fresh solution to old solution would only make things worse. During spring and summer, pour spent nutrient solution around trees and shrubs. Since water does not retain sufficient oxygen necessary to sustain most plants, a means of aeration must be provided, such as a small pump similar to the ones used in fish aquariums. A small, continuous stream of air bubbles should be adequate.
Observation is a key element in hydroponic growing.
The gardener must watch very carefully for signs of disease when growing several plants together in the same container of water. More gardening information is available in the Virginia Gardener Newsletter .