When to clean water cooling system

I'm thinking your decrease in cooling performance isn't due to the coolant looking cloudy, but by radiator fins that are clogged with dust and gunk. Can you see if you have a thick layer of dust and lint built up on the radiator? If it is really dirty, you might need to remove it for cleaning once your loop is drained. Unscrew, remove the fans and clean well under running water, inside and out. Rinsing inside gets rid of gunk trapped, rinsing the fins helps improve airflow for heat conduction. Reinstall as before.

It wouldn't be a bad idea to also remove the CPU block and the pump and clean, if possible. CPU block should come apart with screws, either allen or phillips. Be careful of any rubber grommets or gaskets.

Best bet would be to determine the tubing size (I think XSPC used to use 7/16" ID by defaults, but check) and just get new tubing. (In fact, this is needed...not really optional by looking at your photos). Figure out the best way to drain your system without getting coolant everywhere. Once drained, remove the old tubing and install new tubing.

Unplug your ATX 24-pin connector to your motherboard. Use a paperclip or small wire to jump green to any black wire. Leave unplugged. This will allow your power supply to run your SATA and Molex connections without powering the rest of your computer. Once plugged in, it will automatically be turned ON, so use the toggle switch on the back of your PSU or use a simple power strip to turn on/off as needed.

Install new tubing, secure with clamps, etc as needed.

Fill your reservoir. Distilled water and some anti-microbial works best; you don't need any fancy coolants, but there isn't anyone here to stop you. Coolants won't perform as well as plain distilled water, but some have solutions that prevent growth as well as anti-corrosives. Even a very mild mixture of car antifreeze could work as long as you mix it with a higher percentage of water instead of coolant...like 70/30 or 75/25 percent.

Toggle on the power, allow the pump to push water through the loop. (At this point, start looking for any leaks where tubing was replaced...this is the time to find them...hopefully not later). Cycle power off when the reservoir gets low; do not let the pump suck air...stop it before it empties completely. Refill the reservoir. Repeat these steps until the reservoir remains full and can be left on without the level dropping. Top off with a tiny bit of head space and cap the reservoir. Allow the pump to run for 15-30 minutes as-is. After this time, stop the pump, and check your reservoir level. Add more coolant/water if needed. Turn back on. Tilt and rock your case to help dislodge air bubbles from the radiator. Air will make it's way to the reservoir and displace coolant causing an air bubble or headspace in the reservoir. Be aware that this can also look like you have a leak, since your res level appears to be dropping; just make sure its air displacement and you cannot find any leaks.

Allow the pump to run like this for another 15-30 minutes just to ensure you have checked all connections well. When you feel comfortable, turn off, remove the jumper pin from the 24-pin ATX plug, plug back into the motherboard and start up as normal. Check for leaks, but hopefully you still have zero.

Keep a very close eye on things for the first several hours just to make sure. Allow the PC to reach idle and then load temps, and then sustained load temps...and cooldown. Hopefully you see better cooling performance than before.