I haven't checked, but the coolant modules have a priority field so they use one, then the next and so on. If that could be on rods and tanks, you could fill tanks and rods selectivly as they empty. For the rods, you need an extra generator part which could be wasteful. The tanks aren't really setup to remove are they? do the pipes connect to the tank or the holder? Might have a system were you craft/fill a tank that you use to fill the system. Then you could fill in blocks solving rounding problems with partial fills.