No more Nemo here!

Two nights ago, Girl Terror woke up screaming. When I ran into her room, she sobbingly told me all about a mean shark, who had gotten out of Nemo and was in our house. The shark swam right in to the playroom, and he hid behind the chair! And then when Girl Terror and Boy Terror were playing, he came out at them, and they ran and ran and ran so fast! But the shark chased them all the way to the living room! It was TERRIBLE!!!
I sat with her for a while, and we talked about how sharks couldn't really swim in the house. We have no water in the playroom, so the shark would die. She decided to dream about finger-painting instead, and went back to sleep.
The next morning, we talked a bit more about dreams, and how yes, they can be scary, but not real. We talked a lot about sharks and whales, and the safety of houses and boats. She reminded me to keep locking the door at night, just in case.
Girl Terror seemed to recover just fine. She even laughed about the silliness of sharks swimming in a house. I admit, I was pretty pleased with my handling of the situation. She's a tricky kid, and won't buy the usual "dreams are not real" or "Mummy will look after you" lines. She needs logic and reasoning to keep her safe. I thought we had safely banished the evil shark to the ocean.
Last night, she woke up screaming again. I ran down to her room, and found her huddled under the blanket, terrified. "Mummy, the shark! It was going to eat me!" she howled. I said "But remember, the shark can't swim in the house. And the door is locked, too. Our home has no sharks." "No, Mummy" she said, "No sharks here. He was in Superstore! Coming out of the place where the lobsters and crabs live! And he was chasing us in Superstore!"
Great. Now the live-catch tank in the grocery store is possessed with the evil shark. I have no idea how to get rid of him!