It is very important to determine whether you have lead shot or mercury in the tank. Many hydrometers and hydometer/thermometer combinations have lead shot. Old thermometers had mercury. If the spheres are all the same size, sold and grey, you have lead shot. It is somewhat toxic, partly because of impurities such as arsnic. You should scope out the gravel or sand in the area removing as many as possible.
If the spheres are different sized, liquid and silver, you have mercury. Mercury is now considered extremely dangerous to people and you should not come in contact with it. (I know, as a kid I used to rub it on dimes to make them silver, but I probably lost several million nerve cells in the process.) It is extremely difficult to get all of the mercury out of a gravel bed, but you should. I've had many mercury thermometers broken over the years by stomatopods and it is really a pain.