Ive ran H20 for 7 years and I just use distilled water or a mix of distilled water and this stuff:
NCIX.com - Buy Primochill PC Pure Extreme Water Replacement Coolant 32OZ - Clear - 897337001080 In Canada.
Never had an issue with anything growing in my loop and I have never touched biocide of any kind.
Today a lot of tubing contains a biocide (primochill) and anything silver plated helps as well.