Well, it sounds like you had a justifiable emotional investment in the well-being of that fish.
But it also sounds like you took better care of yours than most classmates, and 6 years is a good, long run.
We get attached, it’s our nature. So no, it’s not wrong to be upset when something we’ve been nurturing dies.
You may not have grounds for bereavement leave from work, but being kinda bummed is entirely understandable.
Nuuu fishy :c
Maintaining aquatic beings isn’t as simple as many people believe.
Goldfish-in-a-bowl being an exception.
I congratulate you on keeping it alive for so long, and that’s probably why you feel saddened by it’s death. Time usually breeds some familiarity, it’s normal to feel a loss when you lose it.
Because goldfish kept in bowls die 20 years young ;-;
I was thinking of getting some fish just last week and decided against it. They are a lot of work and I would hate to a ccidently kill the little guys.
Why would it be wrong to feel sad? What did your parents or peers DO to you?
if you also feel the same way for humans. nothing wrong with feeling for animals too
The best thing about animals is that they don’t waste time thinking about WHY they exist or why they exist somewhere. They just work with what they have and learn from what they see. In that, you and your schoolmates were what zoos call environmental enrichment for your fish. 
He probably did live an engaging life watching you all paying attention to him.
I’ve kept fish on/off for 30 years and can say fish DO interact with people who interact with them. I’ve had a lot of fish who “play” with me through the tank wall in various ways, most usually angelfish, gourami or bettas. They also often play with “toys” if you offer them things like shiny marbles, shells, or floating bobbers. It’s super cute.
That all said, it’s normal to mourn a pet that’s been a part of your life for so long. 
I always hate to hear people say “it was just a _____” because if someone is missing the pet, then it definitely was a special pet. 
There is nothing wrong with feeling sad about a pet dying. It would be more “wrong” if you didn’t care at all.
It sounds to me like you gave that fish a very good home and a good life. I doubt it was bothered by going from the store, to a lab, and finally to your home. I think it was great that you took all the other abandoned fish, too. The fact that you did that, and that you care that your fish died today, proves you’ve got a good heart and a kind, compassionate soul. There’s absolutely nothing wrong with that.
I’m sorry for your loss, but you should be glad you gave your fish the best life it could have had given its origins.
Aquatic fish… Are there non-aquatic fish that I’m unaware of?
Im more attached to pets that i lived with than humans i nevet interacted with
What other types of fish are there?
I still sometimes grieve a little over a cat who died 10 years ago.
He was very big and wild, but incredibly loyal and loving.
I still have scars from when he was young, but as he grew older he worked so hard to keep his scalpel-like claws tucked in and keep his temper in check.
And being so big, he gave wonderful cuddles.
Then one night I was raiding with him on pillows on a stool beside me so I could continually stroke him - he’d had cancer for a while and just wanted to stay close and be touched. For a while I was totally involved in that raid, a difficult boss and we kept wiping, and he was being quiet.
Then I stroked him again and he was cold and stiff.
I’ll never forget him, or that night.
When you really get to know another being it doesn’t matter that they have a different shape, and fur, feathers or scales. Deep down we share a life-force, and to experience this togetherness through a loving, trusting relationship is always special.
Don’t apologize to the internet for experiencing empathy…
…they’ll sense your weakness.