I think the dark color would cause the water to get warmer and exacerbate the prblem. Everyone has their own threshhold of what is OK, but I put a small glug of bleach in the 12 gallon can that holds our water and it stays very fresh. My new scheme is a rubber feed pail with a rock to keep it stable, and i change out the water daily. Saw this in a pix on the board somewhere and it seems to be working out.