Chemical analysis can detect picogram levels of contaminants. We're exposed to toxins in everything we eat, most naturally occurring, but at extremely low levels. The research to establish threshold level effects of the various toxins hasn't kept up.
The mercury in tuna was shown to be partially composed of elemental mercury that is naturally occurring while the methylated mercury, a contaminant that is known to be biologically active, appeared at slightly lower levels than previously thought. Sounds different when it is presented that way, doesn't it? While it doesn't mean you can eat all the tuna you want, it also doesn't mean you can't eat any.
Removing your amalgam fillings will very likely expose you to more mercury than leaving them in place.