Problems with overheating, sorry, totally clueless.

So, I inherited my brother's fancy PC last year. It ran fine, so I left to do it's preset thing, but now it started beeping the overheating alarm whenever I play any game for 15 minutes.

I noticed the pump cylinder is not full to the brim, I think it should be? Is that the problem? The screen says it's pumping, but I'm not sure it is, I can't see any movement. Can that even the reason it's not pumping

Google seems to think I can just top up the reservoir, but I'm not sure I have any of the coolant (since it's opaque I assume it's coolant and not just water). Does it matter if I just add water to the coolant, or do I need to get coolant. And does it need to be all the same or can I get any?