As I have said elsewhere....I had a very similar situation in my aquarium as
@jaylach describes above.
One of my older and original female Pepper Cories had been "off" for a week. She was eating fine but just not enthuasiastic in her activity (the polar opposite to her normal behaviour). The rest of the fish were escorting her everywhere, getting niggly at anyone who got to close to her and generally encouraging her to move around in a very gentle way.
My fish will not be caught under any circumstances....one sight of the net and I suddenly have what seems like an empty aquarium.....everyone scatters and hides. It is fine by me, just cos a fish is domesticated in its breeding does not make it 100% compliant to the whims of its human caretaker.
Anyway, as time went on, this Pepper was slowing down, so I thought maybe I should take her out of the community and give her a rest from the hubbub of the aquarium. She was not having ANY of it. She summoned what strength that she had and scurried off with the others as normal.
So I left her...but kept a close eye on her. The rest of the fish were guarding her and encouraging her...just as that plucky little Tetra is with
@jaylach 's Pleco.
Then one morning all the fish swam away from her and she was up at the surface in one corner of the aquarium. I took the hood off the aquarium, she never moved. So leaving the hood off, I went into the kitchen to get my net and a jug...she still never moved...put water from the aquarium into the jug....she still never moved. The rest of the fish stayed a discreet distance away, didn't hide as usual.
I put the net into the aquarium and she swam and flopped into it. She had passed away before I had chance to empty her from the net to the jug of water.
For the rest of the day all the other fish in the aquarium stayed together, very little activity, none of the usual play or mischief.
If that is not an emotional response from the dying fish and her tankmates, then I do not know what is.
If you watch wildlife films, especially about Orca and other whales, when a mother loses her baby, they carry that baby for miles and miles and the pod reacts to protect, comfort and support the grieving mother.
That is emotion in its purest sense...the sense of loss is an emotion that every living thing experiences, the sense of protecting those who are unwell or insecure is an emotion that every living thing experiences.
That little Tetra can sense something is wrong with the Pleco, the response that it is showing is an emotional response.