A reef tank with photosynthesis which is going on while the lights are on is a major contributor to the levels of oxygen in a reef tank. Normally dissolved oxygen levels are good during the day because of this contribution of oxygen by coral, Live rocks coraline algae and its other possible photosynthesis life forms, micro algae and macro algae if present. These things also remove carbon dioxide as part of this process. A tanks biological filtration system and fish use the oxygen of course and release carbon dioxide. Then there are the other gases released by break down of organics. The major problem lies in that the dissolved oxygen levels drop significantly shortly after the main tank light are turned off and the levels stay down until after the lights go back on. No amount of circulation or agitation of the air water exchange area corrects this. Immediate results are however seen when macro alga is grown in an attached refugium or sump with lighting run opposite of the main tanks lights. The amount of oxygen exchange going on at a tanks surface does not require a lot of air movement, nor does a lot of air movement or water agitation increase the oxygen levels significantly over that provide by some water movement. Of course there should be air gaps for exchange of air between inside and outside of a glass covered tank, however the extent of how much is dependent on whether you also have a sump and or a skimmer. A skimmer in general will increase the oxygen level in your tanks water more than the tanks surface exchange will provide. You still must vent out gases that collect above your tank though. Lots of oxygen is needed by a reef tanks biological filtrations bacteria. There is such a thing as what is called a "bacterial oxygen demand." That is a measurement of how much oxygen is required to break down the organic compounds in a sample of water. In the waste water treatment world a facility is required by the EPA and the states environmental agencies to limit the BOD (bacterial oxygen demand) to certain levels or less before discharging it into lakes, rivers, streams the ocean. The aquarium trade runs a little backward in that it tests the waters ability to handle a BOD without knowing what the BOD level is. The aquarium industry does this through the use of ORP monitors. I can tell you that more information about your tanks system, such as is there: a sump, skimmer, refugium, macro algae, coraline algae would be needed to make any opinion about your tanks oxygen levels anything but a wild guess. However, if your fish are not spending all their time in front of pump returns or power heads during the day then you likely have adequate dissolved oxygen levels while photosynthesis is at work. If at night you go out to your tank a couples hours after the lights are out and your fish are in front of water returns or jets, or at the top of the tank just hanging around, then a sump or refugium with plants under a opposite light cycle to your main tank would be an appropriate action, because even taking off your glass top and increasing circulation would not help sufficiently. Hopefully this long thread helps answers some questions.
