Most of the information on those pages came from the High Seas Beta shard where the contributors of those pages were able to set their fishing skill to 120 instantly. For this reason, your chance to catch one with a fishing skill lower than 120 is a big unknown at the moment.
Furthermore, being on a test shard, we were able to set odd templates that fit dungeon fishing which made us catch those rare fishes with less interruption from the spawn around the fishing spot of the dungeons.
Most of those rare fish are caught by using the logic put forward by
Crysta: Name vs. Spot. So we searched for Crystal Fish inside the Prism of Light cause that's where the crystals are, and Bull Fish inside Labyrinth cause that is where the bull-like creatures, minatours are.
Catching rare fish at the deepwaters of the various facets is pretty straight forward, I dont think you need any advice on that. For the rare dungeon fish, basic methods like luring the spawn away, casting invisibility spell, hiding etc. proved to be very useful.
Some of the locations these dungeon fish are caught are as follows: One of the Crystal Fish is caught on the north edge of the river inside the Prism of Light where crystal sea serpents and crystal hydra spawns around. One of the Bull Fish is caught at the waters just north-west of Miasma. One of the Summer Dragonfish is caught at the middle well north west of the first level of Destard. One of the Reaper fish is caught on the same screen as the bell inside Doom. Winter Dragonfish is caught on the first bridge to the north east at Ice. One of the Fire Fish is caught at the long bridge of the second level of Shame. Most of the Lava Fish is caught on the bridge near the gray goblins, or south of the stairs to the Abyss.
We're still missing information on some of these rare fish, I have to confess that after spending three hours to catch a rare fish in Twisted Weald (which I couldnt but finally others did which was a relief), I pretty much quit seeking for the new ones. There are names at the end of the stratics page, those people slept too little during the beta just to provide those information, so they probably took a break from hours of fishing on certain dungeon spots, just for the chance to catch a rare fish.
Thanks for reminding us that there are still some rare fish out there that are not accounted for. Use
Crysta's advice to find those: Name vs. Spot.