I don't think it would do any harm to use the water. After all, think of the slurry farmers spread on their fields, not to mention the chemicals they use. I am in the process of making my own plant feed. I have put a load of unwanted weeds into a 200 litre water butt, and filled it with water. That was a week ago now, and you should smell the stuff :o What size are the water butts?