Looked online to get more insight as to what the tooltip means by uncovering hiddne fish, and the best answer I could find yet was by a user called msquiltalot on reddit:
"It appears that once you have acquired the Lore book for a region, Fish Eye needs to be used to actually see the fish as it is "hidden". I had to use it at Clearpool in Coerthas West to see the Capelin.
At Halo (the Magma Pool near the entrance to Sohm Al), Fish Eye is necessary to see the Legendary fish there: the Lavalord. I found I was able to catch all the other fish there without Fish Eye at level 60. BTW, you might want to leave SNAG off here as you will catch lots of Magma Tree with it on - although it may have some purpose in the future, it serves none at the moment."