what plant is this?

"idk" means "i don't know"
:)
i think it looks like ludwigia. its stem is a reddish..
 
It looks Ludwigia-ish to me as well.

This is probably a strange guess, but I'm also going to throw Shinnersia rivularis into the mix. I believe they don't develop the "oak" leaves in less-than-ideal conditions, but it looks like some of the top leaves in the second picture have some "oak" to them.
 
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