What is wrong with my fish?

Ok, so my tank's relatively new. It's still in the cycling phase, actually. I've been using a bacterial additive to help speed cycling, and i'm adding fish a few at a time. The ammonia's relatively low (not high enough to hurt the fish) and right now, i have a betta, a blue gourami, five small cories, and well... one cherry barb in there, and it's 29gal. I had five barbs, but four have died... every single other fish is acting completely normal, and the barbs were too, but one by one they've been dying. At first they have, like, these seizures, and then their stomachs turn white and they die. I have one left and it's acting perfectly normal, and so are the other fish, and the water is fine, so i don't think it's the water.... what could be wrong with them?