1) why did u just get new fish when u have no idea what the prob is lol...not trying to call u out, but yea
2) idk if anybody stated this, but GBRs LIKE warm water, around 82F+ which means they are INCOMPATIBLE with tiger barbs, it may not be noticeable now, but the fish(whichever are in the wrong temp) will be stressed...i know some sites state temps should be 78F-86F for GBRs, but I have owned them and known quite a few others with them, and nobody kept tanks lower than 82F...so either you need to rehome GBRs or barbs(or return if possible)...
3) try and do 25% WCs daily for a while, and maybe, if you suspect parasites, dose maracyn(liquid or powder) as a preventive med or actually treatment if they have it...but don't dose until you are somewhat sure or see any surefire signs of parasitic disease...
4) can you try getting pics of GBRs? and what is your tank pH?
5) good luck

hope they make it!