This is bit of an area that many reefs tend to not consider.  It is very important to have sufficient gas exchange to and from the aquarium.  Not only do you have to consider the water movement within the tank, but also air movement over the top of the water surface and air exchange between the room that the tank is located in and the outside.

Constant 

My tank is currently ventilated 24 hours a day by a kitchen / bathroom fan (Ring Grip 25cm) which draws air from the hood and sump area, via two length of flexible ducting.

 Ventilation duct drawing air frm top of tank.

Ducting (not shown) is connected to the two holes at the top. 

Protein Skimmer 

The air inlet for the protein skimmer draws air from outside, ensuring that it is not using "stale" air from inside that may have elevated carbon dioxide levels.

 Protein skimmer air inlet, drawing air from outside.

Stand 

The warmer months of the year there is also a ventilation fan blowing air into the stand area.  This is to remove the heat generated by the lighting ballasts, which are located there, and for the hotter periods assist with evaporative cooling.

 Stand ventilation fan, at bottom, outside view.

Stand ventilation fan, inside view.

Room 

For the summer / warmer months, another two room ventilation fans are utilised.  These are to ensure that the room does not become too humid.  If it does, then evaporation is greatly reduced and that then reduces the cooling of the system.

Room ventilation fans in Summer, open, from the inside.

Room ventilation fans in Summer, open, from the outside.

For the cooler months of the year the opens are blocked over by some plastic sheeting, stopping the cold drafts that will otherwise occur.

Room ventilation fans in Winter, closed, from the inside.

Room ventilation fans in Winter, closded, from the outside.