Over-dilution of formulated glycol heat transfer fluid (HTF) causes corrosion problems. The reason for this is insufficient concentration of corrosion inhibitors and insufficient pH buffering capacity will form acidic compounds when they oxidize which causes thermal degradation. Over-dilution also causes bio-degradation as discussed during the June 25, 2015 Coffee with Caleffi webinar.
All water soluble organic liquids will bio-degrade when suitably diluted with water. It turns out that for glycols the “line in the sand” is somewhere between 20% and 25% volume in water. Less than 20% is a surefire recipe for bio-fouling, whereas 25% or higher will prevent bacteria and fungi from sustainable growth. The high osmotic pressure caused by the concentration of glycol creates bio-static conditions i.e. does not kill bacteria, but is non-sustainable for growth.
Bottom line? Don't do it; keep your glycol concentration above 25%.