The type of mercury in your therm is not very water soluable at all. More dangerous to living organisms are methylmercury-which is bioaccumulative and highly soluable in water-not the kind in your fish tank.
If I really wanted to be say, these things would help a great big:
Remove all living things to a different tank
I would suggest you throw away all your gravel and use new ones from someone else's tank.
Scrub the sump with a Hg removing sponge and glove, clean your aquarium glass.
Put everything back afterwards and start using 1lb of carbon every 7 days.
Drip "Kalkwasser"
Mercury in any form will harm/kill animals, inorganic mercury zero (which is the kind you're dealing with) isn't as bad as it's PR reputation.
1. it don't dissolve well
2. removed somewhat by carbon and calcium hydroxide ("Kalkwasser")
3. whatever dissolved don't pose a big threat to your livestock