What would happen if you made a planet out of fish?

See also, "Blueberry Earth" https://arxiv.org/abs/1807.10553

  This paper explores the physics of the what-if question "what if the entire Earth was instantaneously replaced with an equal volume of closely packed, but uncompressed blueberries?" While the assumption may be absurd, the consequences can be explored rigorously using elementary physics. The result is not entirely dissimilar to a small ocean-world exoplanet.

further reading: xkcd's "A mole of moles" https://what-if.xkcd.com/4/