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my cory sits at the surface for about 10-30 minute periods.

this started about 2 weeks ago

after he's done at the surface he has normal behavior

i don't know if i need to quarantine him and give medication or something else


i have 4 other corys and one pleco that don't have the same problem.

not sure what to do here, help is appreciated.

Hi there,

I don't think you should medicate for this, or quarantine. You say the fish is otherwise fine, apart from when it hangs at the surface.
Check your water parameters. If there was something wrong the other fish would show it as well.
I would do a large water change. Don't feed at all for a couple of days. Then feed quality frozen live foods for a week, no dried.

Maybe it was injured at some point in it's life and and it affected it's nervous system in some way? It isn't normal behavior but I don't think it will ever act normally but will otherwise be fine.

I have a Diamond Tetra, Moenkhausia pitteri, which acts normally most of the time except after it has begun eating. Then it swims normally but will do a couple loop de loops then swim straight again, repeats until it has digested the meal. In between feedings it behaves perfectly normally like the rest of the school.
It is almost like the fish equivalent of an intermittent limping problem. I find it's behavior to be very annoying but it isn't really hurting anything.
I think it is the result of an injury sustain during handling. Like maybe it was dropped on its head. Maybe something similar may be what is causing your Corydoras to behave so oddly?

Could be a little swim bladder problem that he is trying to sort out be re equalising his swim bladder buoyancy. I have found live blood worm though even frozen to be an amazing cure for swimbladder problems.

Check your chemistry levels to see if it nitrates are starting to rise.