When the lights are out the plants absorb oxygen and expel carbon dioxide. The yellow and orange pigments are what are seen in variegated plants. The yellow and orange pigments have no part in the process of photosynthesis. Whenever a plant is not exposed to enough light, it looses chlorophyll and the sickly yellow appearance that occurs is a result of the yellow and orange pigments. Iron is essential to the production of chlorophyll.
Plants lacking in iron also take on the yellow appearance.
The photosynthesis process is what makes the plants grow. Also, higher concentrations of CO2 and high light intensity also increase photosynthesis and plant growth is accelerated as a result. The pH value indicates whether the water is acidic or alkaline. Most plants can withstand pH in the range of 6.5 to 7.4 without any problems. There are some exceptions to this rule, but to a large majority of plants this is not critical.