So I am fairly new to water cooling and have been running my current loop for a bit over a year now with no issues. I am running distilled water with Petra's Tech Nuke PHN and have done coolant changes every 3-6 months or so. This morning I decided to look at my GPU block and noticed this discoloration. Do I need to be concerned? Is this just "natural" discoloration that happens over time? Thank you in advance for your help and please let me know if you need more pictures.
This isn't too bad. The build up seems to be copper colored, but it seems your components are nickel plated. What rads, pumps, and blocks do you have. Typically it is recommended to tear the loop down about once a year and clean out the blocks, pumps, and radiators as best as you can. Metal will corrode to some extent over time no matter what. The longer the distilled water is in your system, the less distilled it becomes by picking up ions from the metal. What I do to help combat this is flush a gallon of distilled water through my build every month. If your loop is set up properly with a valve, you can just crack the valve slightly while the system is running and bleed off some water while you fill the res/pump with the new fluid. This essentially replaces a good portion of the water with new stuff and minimizes the amount of air you introduce back into the system. It only takes about 5 minutes for me to do this. Another recommendation is to take everything apart once a year and clean it/replace soft tubing if you are using it. Make sure you are using soft tubing that has been found not to have a problem with leaching. The only thing to do is to take the block apart and try to tell what the discoloration is. You are using the correct biocide, I would just flush the water and add new more often.
As mtra say is bad at all, but the best you can do is taking apart the whole system, clean the blocks with a brush and the flush the radiator a few times with vinegar to remove any gunk that is inside them .... if is possible use a air compressor