As the subject might suggest, I'm having problems with oxygen depletion
overnight in my tank. There's a lot of plants in the tank (29USgal) and a
fair amount of fish, too. Filtration is swapping from two big-ish internals
(bio+foam) to external canister (still bio+foam) gradually. Since the
plants are now really quite abundant in there
(http://www.velvetpurrs.com/pics/fishtank_10weeks.jpg and
http://www.velvetpurrs.com/pics/fishtank_10weeks_2.jpg) I'm seeing problems
where the fish are obviously out of oxygen in the mornings, and it takes at
least a couple of hours before the plants photosynthesis enough to replace
it. Once they get going, I see a bit of pearling from lunchtime onwards,
but it seems that with that volume of plants, they are consuming (along with
the fish) more oxygen overnight than is good for the fish. Now, I use the
air pump to do the CO2 addition (I have a floramat system) - and to prove it
is o2 problems, last night left the air pump bubbling at the back of the
tank with two airstones. It proved my theory - but the thing is, running an
airstone so it bubbles the surface isn't so hot with the coverglass - it
soon ends up covered in limescale and cuts the light out a LOT.

Is running an airpump at night my only option? Should I move the spray bar
to horizontal to the surface, instead of vertical and mounted in a back
corner? I have circulation at the surface, but only very gentle (I *could*
have put a bigger eheim on my tank, and I got the 2213!). Is there anything
else I could try that wouldn't cause splashing at the surface, but would get
more o2 into the water overnight?

Velvet