If the liquid in the thermometer is red, it is not mercury. In fact, aquarium thermometers haven't been made with mercury for quite some time now. The red liquid is alcohol based, I believe. The metal balls could be problematic, but I don't think they contain lead -- I broke a thermometer in a tank once and immediately scooped out as much of the substrate in the area as I could. I found some more of the balls a couple of weeks later and they were rusted, so that indicates they were made of steel.
And now, having said all of that, I will confess that when I broke that thermometer my very first thought was OMG Mercury!!! Get the fish out! Break down the tank!!! And that is what I did BEFORE googling. Oh well, now I know I can break down and rebuild a 25 gallon tank in just over one hour.