First of all, you have to establish whether you broke a mercury thermometer or a weighted hydrometer. The hydrometer has lead shot in it. While this is some what toxic to marine invertebrates, it is not that dangerous and you should be able to clean up the sand and gravel in the immediate area along with the shot. Take everything out in the location where it broke down to the bare glass or acrylic.
If you actually broke a mercury thermometer, the situation is more serious. Mercury is toxic to humans as well as stomatopods and while it will not cause immediate harm, it will down the line. I'm sorry to say that all government recommendations are that the aquarium be completely cleaned using protective clothing and in most states you cannot legally dispose of mercury in the trash. I know, this seems harsh. As a kid 60 years ago we used to rub mercury on dimes to make them silvery, but mercury is a neurotoxin and even small levels of exposure can cause neural damage. I really don't think you have any choice but to dismantle the tank. If you don't, you risk repeated exposure.
Roy
