Beings? yes.
Sentient? I would argue no. But I could be wrong.
I would think most fish are just a bundle of nerves with instincts.
You are wrong! Fish are at least to some degree sentient beings.
They are most definitely not
"just a bundle of nerves with instincts"
They can clearly feel pain and discomfort (why else would a fish scratch itself when feeling itchy?)
They can feel fear (that's why they flee when threated)
They can feel excitement (when they see you bringing the food)
They can learn and remember things they learned (that's why they know that when you stand in front of the tank with the food they will get fed)
Some can recognize each other (proven in experiments with clownfish)
Some have quite a good long term memory (clownfish can remember details of their surroundings for at least 6 months)
Some can solve relative complex problems (proven in experiments with Odonus niger)
They can feel insecure in a new and unfamiliar environment (you see that pretty much with every new fish you put into your tank)
...
I'm quite sure that many fish can feel some form of loneliness, especially those that in the wild live in pairs or groups. They definitely behave significantly different when kept in pairs or groups compared to single specimen.
Since I figured out how to pair fish up I keep all my fish exclusively in pairs or groups and as far as I can tell the fish are doing much better than I experienced it with single specimen.
I think the times of "fish collections" with one of each should come to an end. It's most definitely an outdated concept and not in line with what is called in German "artgerechte halting" (species appropriate keeping).