A strand of Sphagnum Moss underwater.
My carniverous plants require a huge amount of water, in summer almost a pint a day per plant. I collect rainwater, but the pH is neutral, and algae quickly develop, turning the water green and slimy. The best way to avoid this is to have live Sphagnum Moss in the container. The moss takes up any spare calcium or magnesium, and releases Hydrogen ions, this acidifies the water quite dramaticaly, keeping it clear and preventing the growth of algae, fungi and bacteria (it was, and is still, used as a field dressing by troops) - all ideal for 'bog loving' plants.
