Doug, it's probably not great for the underlay but underlay gets all musty and moldy over time anyways (ask anyone who's pulled underlay out of a room, right Brad
), you can't really stop it from happening, you might just accelerate it a wee bit. But if you can't notice any discoloration or any wetness outside of the stand footprint, I'd be inclined to not worry about it until it's time to take the tank down anyways. Odds are pretty good until that day there will be other "spillages" along the way.
Maybe places like Ram could offer us group discounts for "Frequent Flooders Club" or something...