i5 7600K idling at 45-50*C i have a H100iGTX water cooler.?

The temperatures are not in any way harmful as they are.

The important thing is the maximum when under load on a CPU-intensive program for an extended time. That's when the effectiveness of the cooling system shows!

If it is running too hot (and you are not overclocking):

Possibly too much thermal paste, bubbles/voids or a too thick type?

The stuff should flow out as the heatsink is put in place; a peas-size blob laced in the middle of the CPU should flow out completely.

All that should be left is a microscopic film that fills the imperfections in the surfaces.

Many expensive types are total rip-offs and do not work well, especially the really hard ones. The stuff should be like a very soft grease.

Have you got the H100 pump in mid range? Mine (a non -i version) has a button in the centre of the pump and you can select different flow rates. The number of white segments that light up show the range it is in.

The mid range one is OK for my system, low range allows it to get hotter than I like.

ps. Have you got the fans bringing air in through the H100, or blowing out?

I've built a lot of systems with H80 coolers & a few H100 in the past and I've found that they overall run much cooler with the airflow outwards, so the CPU heat goes out the case rather than if it's pulled in, adding to the temperature of the motherboard and GPU etc.

See below:

Why is that temperature bad? You really only have to worry if its at like 90C.