Plant Photosynthesis and Respiration

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This article describes (simply) the two main processes that an aquatic plant in an aquarium go through during the day and night.

  • During exposure to light plants do Photosynthesis and Respiration
  • During lack of light plants only do Respiration


When you have the light on, plants produce oxygen and take in carbon via CO2 to build more tissue. The result is the pH swings towards the alkaline. The plants effectively make sugar (as a energy source) and store it internally.

Technically: (simplified)

CO2 + H2O + light energy => CH2O + O2

Or if you like:

carbon dioxide + water + light energy => carbohydrate (sugar) + oxygen

Key points[edit]

  • produces food
  • stores energy
  • uses water
  • uses carbon dioxide
  • releases oxygen
  • occurs in sunlight


Plants give off CO2 (CO2 in the water is turned into Carbonic acid (H2CO3)) and so the pH shifts towards the acid.

They use the stored sugar as an alternative energy source to the light to perform the growing and 'burning' this sugar causes CO2 to be released.

CH2O + O2 =>  CO2 + H2O + Energy

Or if you like:

carbohydrate (sugar) + oxygen => carbon dioxide + energy

Even if you have no plants this happens, though to a lesser degree, as we all have algae in the tank and this is a plant.

The amount of KH in the tank will act against this pH change and counteract any massive change. Lack of KH causes pH shock as the animals struggle with potentially deadly sudden pH shifts.

Key points[edit]

  • uses food
  • releases energy
  • produces water
  • produces carbon dioxide
  • uses oxygen
  • occurs in the dark as well as light

Reducing the shifts[edit]

It should be pointed out that the rate of Respiration is much smaller than the rate of Photosynthesis. So during the day there is a net gain of oxygen to the water while at night CO2 is build up. For this reason some aquarists with heavily planted tanks switch on an airline at night. The bubble stream agitates the water surface and provides vertical water circulation, thereby stimulating the release of CO2 from the water and the uptake of fresh oxygen.

This is where a lot of would-be plant growers go wrong. They have an airline with cute looking bubbles during the day, not realising that any CO2 in the water is getting driven out of the water. And as CO2 is a major growth ingredient during lights-on the plants die due to lack of carbon to build upon.