[pcentauri]

I will be making a closed water loop for cooling purposes, that shouldn't require any servicing for at least 4 years.
I have been told never to use pure distilled water in such a loop (by chemical coolant manufacturers), because of the ion-hungry quality of distilled water and a pH of 5.6 because of dissolved CO₂. But somehow I can't believe the water will damage the aluminium cooling block to such an extend that it will cause it to leak.

The 250 ml of in the system water will be approx. 55°C, and the water will come in contact with flexible PVC tubing, and acrylic. Considering this system being free of debris and sterile, and a durable water pump being used, what could be said about its longevity?

[marquis]

Well, my first concern would be with the pvc.  Most pvc has some residual  hcl in it.  And as it heats and degrades, it forms more, which speeds up the degradation process.  It also would make the water acidic.  So the first question is, how much contact area with the PVC do you have? 

I've always been nervous about aluminum in water, too. But I'll let someone else answer that question.