My rainbow shark is about four inches long, including his tail. His "castle" is in the corner, but perhaps the castle is too small for him?? I have added a couple of plants today and am now adding six black neons. I don't think he has even noticed them yet. As I left the store, the lady from yesterday asked me what I was feeding my shark today. She thought she was funny, I didn't. But they did tell me that if I want to bring the shark back I could "trade him in" on something else. He is far too beautiful to trade in yet. I am now on the lookout for another "cave". Thank you all for your input and I will keep you posted on my new additions.