RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Fishing Potion
The Old School RuneScape Wiki also has an article on Fishing Potion.

A Fishing Potion is a potion that can be made with the herblaw skill. It requires level 50 herblaw to make and gives 112.5 herblaw experience. When consumed, the players fishing level temporarily increases by 3 levels.

Fishing Potions are most commonly used when a player is 65 fishing, bringing the player's fishing level to 68 so the player can enter the Fishing Guild, located north of Ardougne.

To make a fishing potion, first add avantoe to a vial of water creating an unfinished avantoe potion, then mix in snape grass. As with most potions, initially 3 doses are made, and the doses decrease as they are drank until the potion is an empty vial.