Water is really the only practical solution for a pumped system. Glycol based antifreeze lubricates as well as killing off any bugs, however i wonder if it may have an effect on the plastics used in the pumps? I have run a water cooled computer for about ten years, and that runs on normal tap water, with no antifreeze, and I've never seen a bit of algae or stagnation in the water, despite all the warnings written on the web about needing special additives etc.
Mercury is used in some fullsize vessels, and being about 13 times the density of water, you wouldn't need to move much to get the same effect. But even assuming you can find a supplier, it's nasty stuff, and you don't want it getting into the watertable if you were unlucky enough to spring a leak.
However I prefer the system David mentions. Don't forget the shiftable weight could be placed in the wet area if space is tight. You can control the weight via a watertight gland or bellows.