With the new setup, using a smaller rad, you shouldnt have much of a drop in RH. Plus is looks like you have a pretty hefty humidifier so it should have a problem compensating for the drop in RH. Moreover, you wont lose as much RH because the temperature of the radiator isn't all that cold. On the AC I used, the rad would get to like -30c in a few minutes, and would frost up really quickly so I put a plastic box over it with some holes in either side and it was much better. So what I'm saying here is, I'd stick the rad in the tank, as gil_za pointed out, the temps will drop quicker and it's also a lot more efficient.