What is magic about the fish in the tank?

There is nothing magic about the fish in VF; they are the magic fish from Fish Tycoon.

Like the rain of Isola and the pictures; they are from previous VV games. LDW has always referenced their previous games. Even the flowers in VF are from Isola and Plant Tycoon.

I just wondered because when you go to buy the fish tank it mentions it coming with magic fish.

Yes, the magic fish that you had to breed in Fish Tycoon.