Hi,
Barley straw appears to control it in ours - although, I was a little lapse over winter and didn't replace it so the weed came back! I've popped a new 'torpedo' in and hopefully it will do the trick before the weed gets out of hand.
It's when the straw starts to break-down (rot) that it releases/generates something - don't know the detail, sorry.
One thing I was told to note though is that you must aerate the straw regularly (monthly?) by pulling it out of the water, letting air get to it, then dunk it back in and repeat a couple of times. This is something to do with allowing it to break down 'aerobically' as apposed to 'anaerobically'? One's good, the other's bad - apparently!
Hope this helps.
