I would siphon all the gravel out and turn all the LR upside down under water so that any mercury would fall out of the LR.
Once all the gravel is siphoned out, you could "pan" for the mercury with a gold pan.
Many many years ago miners would add mercury to gravel from a stream bed, in a gold pan. Then they would pan for the gold. The mercury is very heavy and would stay behind along with the gold. In actuality in this situation, the gold would have amalgamated with the mercury and would be last to leave the gold pan.
I can forsee that you could do the same with aquarium gravel as it is very light compared to gold and mercury.
In any case, I would get the mercury out of your tank.
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