What should I do with the Mutant Carp?

I just caught my first legendary fish, a gold-star Mutant Carp. What should I do with it? I don't really want to sell it, because I can only catch one per save file.
 

imnvs

Local Legend
There are 5 Legendary fish which you can only catch 1 of each, and I always put them in aquariums. Then, late game, you can get a quest to catch a relative of each Legendary. I put those in aquariums too. You can be asked to do this quest multiple times.
 
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