You could just use distilled water, it's deionized (DI) enough.
Or if ya want to make your own, and really good drinking water too go get a nice little industrial Reverse Osmosis machine with a 200PSI pump on it.
You can then run that through DI cartridges to get it to unmeasurable ppm of contaminates, but I doubt you would ever need to if the R.O. is adequately serviced.
Pure distilled water or DI water will both dissolve metals, pure water is a very good solvent. Once you add 20% or more of antifreeze or other coolant mix the water is no longer pure though.
As for Algea, tap water is full of it.
2004 KTM 625 SXC
2004 Yamaha FZ-1
1996 KTM 620 RXC