Originally posted by Hobo Misanthropus:

That video (Which lies because there is no FAST way to level fishing, lmao) probably mentioned this, but keep any Level 1 perch you find. You can use these to build a Fishing skill pond, ideally 30M in length by 2 but no more than 4 wide, cast your line and drag it in, fish will nibble, the speed at which they hook is tied to how many fish you catch, so more fish is faster skill, usually you need 20+ Perch in a pond that size to get consistent bites without waiting before you drag to 25m or lower.

That video actually talks about doing exactly that; the fastest way to level fishing is to... well... fish.

It also mentions that it doesn't matter what kind of fish you're catching, as far as "skill-up" is concerned; since low-level perch bite the fastest and are the easiest to reel in, stocking your "fish farm" with them makes perfect sense.