We have five danios in an established 34-gallon tank that has been operating for several months. I'm sorry I don't have water parameters right now but I can go to the fish store to get them if need be. One of the danios has been acting strangely for the past 2 days. Yesterday, he appeared to be backing up to the intake grate for the filter over and over, then "swimming in place" against the current. It looked purposeful but it could be that he was being sucked toward the grate and trying to get away. This morning, he picked a spot toward the bottom of the tank behind a cave and is swimming back and forth in the same spot. But when I opened the tank he rushed to the top, then joined in with the rest for a little fish flake snack. As soon as I closed the tank, he went back to the same spot near the bottom and started the back and forth swimming in the same spot. He almost looks anxious when he does this. Occasionally he goes back to the top and swims against the intake flow again then goes back to the bottom. Does anyone have any ideas on what could be causing this strange behavior and what to do? I'm afraid there's a second one that may be starting up the same behavior. Thanks for any help you can give me.