croc :
I think you'll find that what you get from your grocery is done by osmotic process. One should be able to get true distilled water from the local pharmacy. If one has a clean system, then pure distilled water should not grow any algae at all. If in doubt, then a few crystals of cupric sulfate should do the trick.
Distilled water sold at stores is distilled. Just like the stuff at a pharmacy. My distilled is bought at a pharmancy that's on the shelf next to the drinking water jugs. If it says distilled it's distilled. There are purer waters used in labs for clean room tests etc. Distilled is done by vaporizing the water. It's not the same process as osmosis, thats an RO method. You can't sell RO water as distilled, thats illegal marketing.
As soon as you open a bottle of even $500 gallon pure water in a non-clean room environment, it's not pure anymore. Dust, microbes, chemicals in the air will diffuse into the water.
The reason we run distilled, de-ionized, or 'pure' water is all the hard water chemicals and minerals are removed. This prevents buildup on internal parts. We don't want that hard caked stuff you can usually find on a faucet etc after years of use.
Life is a funny thing in the world of microbes. You put one in a water solution with heat (nice n warm in a WC loop) and some light and it will, as nature has proven, grow. So you need some sort of biocide that is compatible with the closed WC loop. 99% of the aftermarket fluids like Fluid-XP have biocide in them. Could be one drop of stuff, maybe 10. Distilled water doesn't, so we add something.
Cupric sulfate might do the trick. It's the exact same stuff used in Petras PT-Nuke. It's also known as CuSO 4, Copper (II) Sulfate
Whats funny is I have seen results of someone putting the whole tiny bottle of Petras PT-Nuke into a loop (reading instructions is a good thing, supposed to be 1-2 drops) and 3 months later the copper CPU block was clogged with copper growths leached onto the copper channels in the block. So too much of a good thing is bad.