Dust, soot, pollen and other stuff that blows in the wind can settle on the roof and get washed into the rain barrel, especially if it hasn't been raining regularly to keep it clean. Also bird and squirrel droppings, bugs, bits of shells off the tree, half-rotten bits of leaves... If it's anything like my rain barrel, I wouldn't drink it on a dare. In small quantities, well-diluted, that stuff probably isn't going to hurt the fish, but it wouldn't be my first choice. Too many unknowns.