What type of barbs as the ones I looked up are fine with a pH from 6.0-7.0, the easy and best way to stabilize your pH long term is to put crushed coral In your filter, macro algae or limestone, there's many ways to raise the pH but you have to do it slowly so I wouldn't use anything that boosts it fast as that will kill your fish from the drastic change in pH, to do crushed coral get a mesh bag put the crushed coral in it and stick it in the filter, if you don't have room for it then put it in the tank, as for the barbs if you're using something that raises the pH fast that might be why they're dying.
What are the water Parameters as far as ammonia, nitrite, nitrate, kh/gh