WoWDB

Currently the only blowfish pet available in-game. While there are several fish pets, and more to be added in 5.2 from fishing in various Pandaria areas, (source: http://www.elsanglin.com/news/5-2-tiny-carp-battle-pets.html) this model is entirely unique. Perfect pet for unique pet collectors and those who need a strong aquatic pet on their battle team. 

This is a Trading Card Game item, obtained by the Bloat the Bubble Fish card in the expansion "Throne of the Tides."

When summoned it has a water bubble around itself, wouldn't want a fish to die would we. For use as a battle pet Surge Spiked skin and whirlpool are quite useful.

For an all underwater team I suggest Fishy from the jade forest quest, and Sea Pony from darkmoon faire fishing.

The pet itself is able to be sold on the Auction House if you wanted to. You can do this by learning the BoP pet itself from Landro Longshot, opening your pet tab, right clicking on the pet and putting it 'In a Cage'. This turns the pet into a battle pet and allows you to sell it on the AH.

1400/325/260 @ lvl 25, paired with a unique moveset it makes a nice adition to your collection, and due to its aquatic type+ attacks, a strong pet vs most elementals (pandaren air spirit would turn it into a dog eats dog fight as they both have attacks the oponent is vulnerable to)