The Blobfish is a fish that can be found only in the submarine ride at the Night Market, which takes place at The Beach during Winter 15-17. Alternatively, it can be found by using Magic Bait in the South-western corner of the beach: From the left-most pier, in the southwestern corner, fish facing west.
Blobfish can be placed in a Fish Pond, where they will reproduce every 4 days. The initial pond capacity is 1 fish, but the capacity can be increased to 10 by completing four quests. The primary output is beige Blobfish Roe, but more valuable items are possible starting at population 9.
↑The input data in Fish.xnb specifies upper/lower size values of 8 and 25. However, this fish can only be caught in the Submarine, where the maximum possible Fishing Zone is 2. As a result, even with a Fishing skill of 14 and a perfect catch, the largest possible fish size is 20 inches (which happens to be just barely large enough to be iridium quality).