Tiger barbs do the whole hangin with heads down for a few reasons...Usually its for sleeping...but they will also do it when they are overfed...and when there is something wrong with your water...Tiger barbs are especially sensitive to high nitrite levels and will hang with there heads down also when your water isnt acceptable for them. Test your water...if everything is fine then maybe its just eaten to much...and if thats not it...hes lazy HIH