It doesn't look like odd behavior. Blue tangs have a tendency to keep a person on edge with their swimming behavior. Especially when the go to the sand bed and lay sideways mimicking a fish in distress. And at night time, watching your tank with the lights off can also worry a person, because fish behave differently, especially when they are sleeping. One question, how much flow is in the tank? Asking this because, in the one video, the tang looks like it moved swiftly with it actually trying. So you don't worry too much, research or watch videos of your fish and normal behavior. If I were you, and you will not regret it, try and get some inverts if you are able. I have inverts and love them more than the fish in the tank. My favorite is my pair of Skunk Cleaner Shrimp! Oh, and I always try and buy them in pairs. They behave and interact with one another. Other ones I have, and I have a bunch: Coral Banded Shrimp, and Camel Shrimp. Crabs( I don't pair up): Sponge Crab. And a fun sea star to get would be a Chocolate Chip Star. Post anytime you have a question, or just Pm me. Check it at least a few times a day. And I will give you my best opinion possible!