Put the OSRS Items egg inside and wait until you have a fish (you can't put the shark puppy in it.) You may have (from rarest to least rarest) Sharks (rarest): They're the ideal helper in underwater fishing. They allow you to catch fish underwater faster depending on the point. Stages are pup (there isn't shark eggs), juvenile afterward mature. The next is that the Angler Fish. It's the only source for light in dark underwater cave, as candles or lanterns would obviously go out. They have three phases, egg, larvae and subsequently mature.
Pufferfish will be the next. Their eggs are found on the surface of the water that you can access from climbing up the anchor . Climb up it again to visit Captain Murk. Get a pufferfish egg and then incubate it. It will hatch soon. The stages are egg, larvae, juvenile then mature. The most common fish is that the salmon. They allow you to find eggs. When you're in 10 squares of eggs, it will float in circles.
All fish need to keep submerged. You can not head from the water with it following you or on your stock. You can only have one fish at one time even if the fish is using the tamer. Why you cannot put them in the bank is since you need to give them to the Fish Tamer.
You might also have a home submerged in Mogrin. Your dungeon will be temporarily removed until you move your house to a place that is not underwater. Having a house in Mogrin unlocks a brand new feature for the PoH. When you are in Mogrin you can add an aquarium into your PoH. If you move your house, it will still be there and you can update your aquarium items but you cannot make another aquarium until you move back to Mogrin.
To create an aquarium, then you must have level 54 structure. When you put in your aquarium, you will understand a hotspot that looks like a massive fish tank. To make it you should have level 54 structure, 3 regular planks, 8 claws and 4 molten glass bits. In this you'll have 5 small fish or a single medium buy School RuneScape Gold sized fish. For them, go to the Fish Tamer. Sorry folks, emergency, gotta go, I will update more later.