how big is your tank and how many of each fish?
how many tiger barbs did you start with?
how many do you have now?
how long have you had the barbs?
do you have test kits? if so what are your readings for ammonia, nitrite and nitrAte?
did any of the fish show any symptoms before dying?
anything else happen recently that may have stressed them?
i think someone on this site had an experience where their tiger barbs picked on each other to death until only 1 was left :'( i'm not sure it was someone here but i know i saw it somewhere so that might be whats happening
if you can answer those questions and let us know more about your tank it might help
i dont know if the plants & substrate would cause problems unless changing the substrate upset your biofilter, since you have plants i'm guessing you don't have a ugf but maybe it would affect it in some way maybe someone else knows, i sure don't
sorry bout your fishies :'( hope things get better
--liz