Barley will foul water, any decaying plant will. It will raise nitrates and phosphorous, etc.
It doesn't "clean water" in an aquarium, one compressed barley pellet is the same weight as a handful of the pond barley anyway. The way barley is used in ponds, is as the lignin in the barley is consumed, it releases hydrogen peroxide, which kills algae, doesn't clean anything
I used to think the same thing, but it causes nitrate spikes and phosphorous just like every other dead plant! (and had no effect on my algae woes either)