how long ago where the barbs added? and where they Qted first ? while i agree it was likely the cleaning.. since you did not do anything out of the ordinary, ide cross off a few other potentials before pointing fingers. as long as your ammonia stays less then 4ppm i wouldnt do a water change, unless your other fish are showing stress signs... bacteria need ammonia and o2 too thrive, but past a certain point they dont. not to mention exposure to high levels of ammoniavwill damage your fishes gills.