Decompression station

Sorry for the stupid question but what is that tank used for exactly? It looks like it has a hose going into the pvc pipe hung down.

My guess would be its a deco bottle. Probably o2 or a high o2 mix for accelerated deco

As far as i can remember (it was a long time ago) its just a spare cylinder of air in case any of the divers through one reason or another ran low and couldnt finish their deco on their own gas. Typical stop times ranged from 20-30 minutes.
Its not actually part of the deco trapeze - merely clipped on.