My barbs do that too....and every tiger i've owned before these have also done it...I would put it as normal behaviour
I get up during the night quite often and when I look into the aquarium my tiger barbs are nose down. But i've also had problems with that during the day and sometimes with a little color loss (black stripes dull a little and then later get darker again). I read somewhere that when they eat they get a lot of air in their bellies causing this. So now I soak their food a few minutes prior to putting it in tank. They haven't been doing the head down during the day anymore
Juan and Sylvia
I read that when the tiger barbs point their noses down that there is a high level of nitrate in the tank and that it can be corrected with a water change. I noticed this behavior today and I did the weekly water change. But now two of my tiger barbs are staying at the back of the tank. one behind the rocks and one behind the filter. The third tiger barb is fine swimming around the tank on his own. I only have 3 and there aren't any aggressive fish chasing them. Why are the two barbs hiding?