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i picked up a solitary cory. delphax from my LFS. they had no others, and said no more were going to come in, so i took the poor guy home.
he's been settled in for the past five months+ now, and is fine in every way; healthy, growing. he tends to hang out with the elegans i have.

now a couple of months ago we started hearing this strange bubbling/ clicking noise coming from the tank at night. its in the bedroom, and was loud enough to wake my husband at three in the morning.
only recently we've discovered what it is;
its the delphax- he comes to the water surface and pushes his mouth out of the water and makes these clicks and bubbling. its always at night, never when we're out to see him.

none of the other corys are behaving strangely; water quality is always perfect- pH 7, no nitrite, very low nitrate, nearly zero. oxygen saturation seems ok, no one else goes to the top.

could he be in breeding mode? we say he's a he, i'm 99% certain from body shape and just general looks.
or does anyone know if its something more sinister?

It could be just coming up for air.

thats what i would have assumed, but he stays around for about ten seconds; flapping around, causing a kerfuffle!
theres about 25 corys in there with him, none have done anything similar, and he's only started doing it recently...

we've got an air pump going on the tank; its been there well before he was introduced.
in the past the corys would come up for the odd gulp; more frequently when they're breeding.
its just this seems out of place... plus he's really loud!

Maybe it's looking for scraps of food that it can taste?

I've seen groups of corys in the LFS go to the surface and turn upside down when flakes gets tipped in, and they make a bit of noise similar to what you describe.

I usually hear it when I've fed freeze-dried food instead of frozen. It doesn't sink well. The Cories need to come to the top. It's loud, but I love it.