Due to wanting CO2 injection in my planted tank, I designed my trickle
filter to be fairly airtight to cut down on CO2 loss. But a thought
occured to me. The efficiency of a wet-dry filter comes from the high
oxygen environment, correct? So how does oxygen get into the
bio-chamber in a sealed unit? A bit of air gets gurgled down the feed
pipe, but does it also need some vent holes in the lid? BTW, the
bottom of the chamber is sealed since it is below the sump water
level.

Another thing, a popular setup is to have a hose connected from the
siphon to the venturi on a powerhead to extract any accumulated air in
the siphon, and prevent it breaking. I was wondering, since there is
already a pump operating sump to tank, wouldn't it be ok to connect a
venturi to the tank return line and use that to remove the air? Since
my spray bar is totally submersed, how would that be different to
using another powerhead? At least that means one less device to break
down. The failsafe will always be operating whenever the pump is as
well. It could also be designed as a siphon break when the pump is
turned off.