BB's
The BB's are steel i had the same thing happen all they will do is rust. the iron oxide can be toxic to the fish but it would take weeks to have any affect on them. if it was mercury it would have flowed through your UGF to the bottom. the mercury in a 4" thermo. is about .75 grams even if you were to eat it you would injest less that you would eating fresh fish once a week for 2 years. .75 grams dilouted into 55gal of water would me about 125ppm though to a normal person that wouldent be anything. Methylmercury is a potent neurotoxin, causing axonal demyeliniation. Adults can experience symptoms months after an acute exposure consisting of ataxia, blurred vision, hearing deficits and paraesthesias. Fetuses are particularly sensitive to methylmercury, as shown by the more than 1,400 infants from the Minimata area of Japan who were acutely exposed to the toxin in utero when their mothers ate fish contaminated by a factory discharge. The children, while often normal at birth, developed abnormal reflexes, problems with suckling, swallowing, gait, and speech, and mental retardation. though for it to be a merc. thermo. it would have to be more that 10 years old and the fluid that you read temp would be silver not red. hope this helps and also informs. though i dont think you have anything to worrie about
P.S. also i wouldnt let my wife do anything with her tank while she was pregnent either.