Many years ago I was a member of a club that stocked a mature ex-sandpit with several river species, these included barbel, chub and grayling. The barbel were caught on a fairly regular basis for a few years, as were the grayling, the chub thrived and grew to 7lbs+ but the other 2 species eventually seemed to disappear.
I totally disagree with the stocking of barbel into stillwaters, though I know it's been going on for years, there was a complex of pits in the north London area, Lea Valley? back in the 70's that had doubles in.