Not saying it can't happen, but on a tank with healthy water parameters a fish should never die from lack of oxygen.. unless it's grossly overstocked,
Pearl gouramis are "Peaceful" I had 2 of mine die to a funal infection they had upon purchase, now I have 2 left and one constantly bullys the
To bad you don't live closer. We have them at our LFS. young ones at 2" for $6-$8 at most stores. Would be worried about temps this time of year
its a stressed young JD.
I currently have a 120g thats overfiltered and currently has a submersible pump with a L bend above it to create surface agitation and aerate the water.