Note to self: WalMart distilled water ain't exactly the cleanest stuff in the world.
I had a bacterial or algae bloom in my computer's cooling circuit, and it got progressively worse over the course of a couple weeks. The reservoir looked like a kitchen sink full of dish soap suds as the coolant foamed and actually pressurized the tank.
A quick Google of the topic, and it appears I've gotten a biological mess in the system. So it all got drained, and a diluted vinegar/Walgreen's distilled water flush was performed, followed by a pure distilled water rinse. I saw no sludge visible in either the reservoir, hoses, or CPU water blocks, so hopefully I got rid of most of the stuff.
Of course, that meant that my $14.00/700ml Koolance blue coolant was gone for good, and it would take a few days for another bottle or two to show up via the brown UPS truck. So I grabbed the big container of Prestone DexCool and another bottle of Walgreen's distilled water to make a 50/50 batch of automotive antifreeze. Into the reservoir it went, and I power-cycled the system until it was burped adequately.
To my relief, there's no more foaming, and opening the fill plug in the reservoir no longer results in an audible pressure relief. Temperatures are just fine, even with the thicker solution. It definitely looks different with the orange/red stuff running through the hoses and reservoir, especially with the blue LEDs back-lighting the tank. Call it a lesson learned, but I used my remaining jugs of WalMart distilled product to water my tomato plants.
Koolance says to change out coolant once a year. I made it all of a whopping three months, but I also bought a used system, so there's no telling how long it had been running without a change. I'll definitely pay closer attention now.