Nitrogen: dissolved dinitrogen
Dissolved dinitrogen. Dissolved oxygen and carbon dioxide in the water have controlling effects on the aquatic community. They are essential to all the organisms in the aquarium, and I'll get back to them in the next two pages. Dinitrogen (N2), the gas that makes up almost four-fifths of the atmosphere, is so inert that it has no direct effect on freshwater fishes; the knack of "fixing" this nitrogen, cracking the molecule and using the freed N in metabolism, was solved by some bacteria ("diazotrphs") early in earth's history, and the technique has never much caught on. In the nitrogen cycle discussed among "nutrient cycles" in the menu at the left, a nitrogen atom is always already combined in a molecule, whether ammonia or nitrate.
But atmospheric nitrogen gas is invisibly dissolved in all waters too, though it's not particularly soluble, compared to oxygen and carbon dioxide. You'll recall that when deep-sea divers surface too rapidly, the nitrogen that has become dissolved in their blood at great pressure can come bubbling out of solution, causing the agonizing pain and possible permanent damage of the decompression sickness called "the bends." Nevertheless, the dissolved gases that most affect the aquarium are the reactive ones: oxygen and carbon dioxide.