Diagram of the <a href="https://en.wikipedia.org/wiki/nitrogen_cycle" class="extiw" title="w:nitrogen cycle">nitrogen cycle</a> in an <a href="https://en.wikipedia.org/wiki/aquarium" class="extiw" title="w:aquarium">aquarium</a>.
<a href="//commons.wikimedia.org/wiki/User:Ilmari_Karonen" title="User:Ilmari Karonen">Ilmari Karonen</a>
2005-10-20
<span class="int-own-work" lang="en">Own work</span>