Coolant System
My machine was supplied with a (pond) pump and I obtained a 55 litre tub on eBay. No particular reason for it being that size except that it was easily available.
Distilled water is recommended for the cooling system but I couldn't find a supplier. In the end I found an aquarium shop, 30 miles away, that sold "reversed osmosis water" a.k.a. "RO Water", for 15p/litre. I took my tub across to them and got them to put 45 litres in it (so it's not completely full). Remember that water weights 1kg per litre so 45 litres takes some hefting.
My tub (which had previously been used for the bulk transportation of olives), came with a screw top lid and the plastic insert/cover thing that you see in the photo to the right. As the photo shows I cut into this so that I could feed the tubes and the wire through the pumps. This prevents evaporation and contamination.
I did not add antifreeze or anything else to the water. My setup sits in the corner of my front room so it isn't going to be subject to cold. Additionally, as the tub is dark blue plastic and positioned in a shady spot, I don't anticipate any problems with algae.
I haven't noticed any change in water temperature when using the machine. I think it would take quite a lot of constant usage to warm up 45 litres of water and my normal pattern of use is that I'll spend a couple of minutes cutting something and then go off for several minutes to modify drawings, assemble things, etc. I guess I could manage with a lot less than 45 litres of water but as I said, the 55 litre tub was easily available to me.
Flow Indicators
The only thing I'm not happy about with this aspect of the system is that I have no indication that the water is actually flowing. I have wondered, if the pump died, whether I would be aware of it?
I guess the machine would sound a bit different when I switched it on, but that's it. Not a lot to go on and of course it is important that it is working correctly. I did think to have the return tube to the reservoir out of the water so I could hear it trickling however this results in air being reintroduced to the laser tube every time the machine is switched off. Not good.
I asked on here whether anybody had any bright ideas and was pointed in the direction of sites that sell products for modifying PCs. Apparently some (crazy?) people install water cooling systems into their PCs and the aforementioned sites sell flow indicators. I haven't got one yet but watch this space. ;-)