The Fish Tank back bling can only be obtained by purchasing the legendary outfit called Leviathan for 2,000 V-Bucks. After purchasing the outfit, this back bling will be available for the player to use. Although it is designed to be used alongside the outfit, the Fish Tank back bling can also be paired with other outfits.
Fish Tank high-quality PNG images with transparent background to use them as you wish (YouTube thumbnails, skin concept and so on).
Used in-game to Shop and Locker, credit to Epic Games.