The process by which plants obtain energy from light. Light energy is trapped by specialised cells within leaf tissue and used to break apart the bonds in water (H20), creating hydrogen (H) and oxygen (O2). Carbon dioxide, hydrogen, and oxygen then combine to form glucose, a basic sugar and important source of energy.
Some oxygen is left over from the process, and this is released back into the water and atmosphere. It is important for fish keepers to recognise that at night plants do not photosynthesise and actually use up oxygen. Although during a 24 hour period plants may produce more oxygen than is used up, this is quickly released into the atmosphere. In contrast, at night, oxygen used up is not as readily replaced; so dissolved oxygen can drop to dangerously low levels for fish and larger organisms when a large number of plants are present.