Reaching level 99 on Runescape fishing skill is one of the most tedious objectives to reach (besides Runecrafting), but the accomplishment is also very rewarding. This article could even make you up to +1m depending on the methods you choose.
Net shrimp at Lumbridge Swamp at levels 1-20. You'll only need to catch about 100 of them (15-20 minutes). You can drop them for faster experience, or if you need GP for more bait/feathers at higher fishing levels, bank them in Lumbridge.
Bait fish at the river next to Lumbridge from levels 10-25, or if you want a close bank, go to Draynor Village. It'll require that you buy about 360 fish bait, which you can find at the Port Sarim or at Grand Exchange for bait and fishing rod.
Buy about 1k feathers and head to Barbarian Village for levels 25-50.
Fly fish from levels 50-99 (184,759 salmon). You will obtain a 12.8m profit doing this.
Carry on fly fishing until level 70 or 99. You will earn less money if you continue to fly fish until level 99, although you get more experience if you start to drop them which is also known as Powerfishing.
Monkfish if you choose to stop at level 70. That is one of the easiest strand, and it gives 120 XP per catch. Before fishing monkfish you must have completed Swan Song. After the quest start fishing them at around level 75 up until level 99. This is also one of the best ways to make money.
Ever since Rocktails were added to RuneScape, they have been very popular to fish. You can fish them at level 90 for good money at about 38k XP per hour, and they fish at about the speed of sharks. World 84 is suggested to fish these as it is the unofficial world for rocktails. You can also fish cavefish at level 85 for about 25 - 30k XP per hour and decent money.
Leaping Sturgeon/Salmon/Trout. Known to be the best fishing experience, this requires Barbarian training, which can be started at Otto's Grotto. At level 80 (which is the recommended level to fish this at), each full inventory takes about 2 minutes, each giving about 1500 XP. However, to fish all of these, you need at least level 45 Agility & level 45 Strength.