Never changed the coolant. Over time (months) it will become less a bit and i fill it up. Checking the water
all 4 to 8 weeks. I would go for DP Ultra coolant with a color. (i have always green)
This will help in case you have a leakage.
I use distilled water with ~5% of antifreeze in it to prevent growth. So far so good.
Yeah, I'll get a similar one soon. Does it require external power or works only from HDMI/USB?
Under construction:

LOL this setup reminds me of something

7 cards are already in, are you planning to add 2nd distribution block? What quick disconnects/angle fittings are used, are these Alphacool (since you're in Germany that's be my first thought)