The fastest fishing method in the game is 2tick swordfish where you can get up to 135k fishing xp per hour.
This requires you to be attacked every 2 gameticks, so for the best efficiency, youll want to do this in the fishing guild for the fishing boost with an alt. Although, you can still do this without an alt in the piscarilius house on zeah.
2-tick swordfishing is pretty easy but does need your full attention. You simply click on the fishing spot as soon as the hitsplat appears on your character.