what symbol? There is a story of Krishna capturing a fish and releasing it (can’t remember the entire story) but not of Krishna being a fish.
This version has Shiva manifesting himself as a fish and rescuing Ma Nu. Both are messianic manifestations of the same god-head (which can be a single god or a whole committee of gods)
There are a number of videos available on the net so you can actually experience the event ~ plus watch Ma-Nu himself save the 7 great sages through his clever use of a boat.
Two things ~ the universal imagery of a man, his boat, a great fish, a great flood ~ these appear in various arrangements in many ancient stories. More importantly Ma Nu has the same numbers, 1 man, 7 sages, etc. Then there's the snake used as a rope with his 8 heads, etc. You can line these numbers up against other ancient versions of the great flood, and you see they the same ones.
That suggests that behind this meme there's a widespread shamanistic tradition that spread among all the trade routes developed toward the end of the Ice Age. The tale has staying power.