Karamja Fishing

Fishing starts becoming Profitable at level 35 for F2P, buy a Harpoon from the Port Sarim Fishing shop, and Take a ship to Karamja. Note : If you do not have the Pirate's Treasure Completed, you will need to pay to board the ship. There is usually alot of people at the Karamja fishing point, so be prepared to hunt for your Harpoon fishing Spots. 

Cooked Tuna is 138 gp Each, since you cannot fish anything else for a good profit in F2P, if you fish tuna until level 99, you'll need about 13M Exp, so if tuna is 60 Exp each, you'll make about 30M by the time you get to fishing level 99, on the island there is a man that will note your fish on the island, so banking is not neccesary.