@Upright Man. Thanks for your comprehensive response (which I won't just quote in full & be lazy).
The system you describe with an external reservoir differs from the designs I found from my own research, but I guess there's more than one way to skin a cat... Yours seems to be "drain to waste", rather than recirculating the nute solution (which is the concept I found from my own research after you mentioned it).
The reason I was asking/assumed about the reservoir INSIDE the tent & water temperatures is that I also have hot summers & freezing winters in Vic. The idea of a thermal mass during winter appeals to me, as it means stability when it's bloody cold & I grow outside in the ground or fabric pots in summer. That thermal mass also means I can use it to help get seedlings started ahead of the OD season.
BTW, this isn't just about weed. It's as much about my chilli, tomato & other food crops which will benefit from an early start, given that they need more heat to germinate than I can provide in ambient temperatures.
Thanks again for your response & leading me down yet another rabbit-hole....