Can't see it would do anything but purify the drainoff. Might cause problems if you check ppm and ph of your runoff. Be interesting to hear other opinions.
No. rocks or stones will give the aeration & weight the bucket so the wind won't roll it to the local Pig Feeding Lot- Donut shop. Just confirmed doubts. The normal charcoal we use is chalk-full of chemicals. If its 100% hardwood ashes, a little is cool.