Put the egg inside and RuneScape gold wait till you have a fish (you can't put the shark pup in it.) You may have (from rarest to least rarest) Sharks (rarest): They're the ideal helper in submerged fishing. They help you catch fish underwater quicker depending on the stage. Stages are pup (there is not shark eggs), juvenile then adult. The next is that the Angler Fish. It's the sole resource for light in dark underwater cave, as candles or lanterns would go out. They have three stages, egg, larvae and adult.
Pufferfish will be the next. Their eggs are located on the surface of the water that you can access from climbing up the anchor rope. Climb up it to go to Captain Murk. Get a pufferfish egg and then incubate it. It will hatch shortly. They blow up when an enemy attacks you, poisoning the enemy. The most common fish would be the salmon. They help you locate eggs. When you are in 10 squares of eggs, it will swim in circles.
All fish need to stay submerged. You can't go from the water following you or in your stock. You can have just one fish at a time even when the fish is with the tamer. Why you can't put them in the lender is because you need to give them into the Fish Tamer.
You may also have a home submerged in Mogrin. Your dungeon will be temporarily removed until you move your home to a location which isn't underwater. Having a home in Mogrin unlocks a new feature for the PoH. When you are in Mogrin you may add an aquarium to your PoH. When you move your property, it will still be there and you'll be able to upgrade your aquarium items but you cannot make another aquarium till you move back to Mogrin.
To make an aquarium, then you must have level 54 construction. When you put in your aquarium, then you will understand a hotspot which resembles a massive fish tank. To make it you must have level 54 construction, 3 regular boards, 8 claws and 4 molten glass bits. This way you'll have 5 little fish or a single medium sized fish. To get them, go to cheap RS gold the Fish Tamer. Sorry folks, emergency, gotta go, I will update more later.