this thread is kind of old but im going to leave a reply in the hopes it helps in some way. unless i misunderstand the tank has no filter? this will cause low oxygen and poor water quality. tiger barbs swim in place and point their heads down when there is low oxygen. the most likely reason you have problems is aggression between the tiger barbs. the tiger barbs try to find the weakest in their pack (the most dominant eat before the rest so the hierarchy is very important to them) theybwill have brutal fights when deciding the weakest allot of the times the tiger barb will die after the fight due to being an injured stressed outcast unless the tiger barb is very healthy and can recover. allot of the times the tiger barbs will eat eachothers scales off in these fights making good water quality essential for their survival. tiger barbs need some of the largest schools in the aquarium hobby in my opinion the minimum for a school is 15, for most schooling fish like tetras id recommend 6 as the minimum. if you keep a school less than 15 the more the better if you keep 5+ they will slowly pick eachother off but not as fast as lets say 3. if you have 3 2 of the three could easily die overnitght from fighting and youll be left with one that will become very stress due to being alone. if you have no filter atleast get an airpump or lots of plants so the tank can build up a natural filter.
