A while ago my custom loop started very slowly dropping water levels. The loop consisted of 2x Thermochill PA 120.3 radiators (pretty ancient, 9+ years old but always looked after) and a Laing DDC Ultra 18W. The fluid was distilled water and a kill-coil was used to inhibit any growth. I checked it many times to try and isolate any potential leaks and never found anything, so I kept it running but kept an close eye of the fluid levels, which indeed continued to drop (the system would lose about 150ml of fluid over 2-3 months, so it seemed it indeed had a small drip-leak. Eventually I said enough was enough and did a pretty extensive leak-test with the components off, and probably because the loop was now cold I found signs of evaporated water drops on a hard-drive bay, so luckily the system leak had not occurred near any components. I then did a test and sure enough the top mounted Thermochill radiator had a very small drip leak that had obviously worsened over time. So I redid the loop, removing LQ for the cpu and just using the 1 good radiator for my GPU, and I decided to use Distilled + EK-Cyrofuel for the new loop, removing the kill-coil. The water itself of the loop was clear, with a bit of black tarnishing on the copper blocks, but nothing particularly noticeable. The strange part is I isolated the leak point in the radiator to inside the fin-pipes. At first I thought I might have punctured a pipe with installation of radiator fans/screws, but it turned out the leak coming from deep in the middle of the radiator away from any screw holes. The leak only occurred with the high-pressure generated with the pump-loop running, so I was basically unable to locate the exact location of the leak once I removed it from the loop. Has anyone had anything like this happen? The only theory I really have is some sort of galvanic corrosion that slowly ate away at the internal radiator fin-pipes, but I am not sure if such a thing is even really possible.