Assuming you are talking about the nut which secures the flush syphon to the tank, standard bathroom silicon is fine. But don't buy the cheap stuff, get acetone based silicon (usually badged sanitary silicon). This stuff smells of pear-drops and nail polish remover, is not water soluable (like normal bath stuff), is a pain to get off your hands and anything else you get it on, but lasts forever.
Unbolt your tank from the pan (assuming its is a close coupled) and unbolt the syphon, stuck a good smear around the thread, both sides of the tank and tighten back up. Then put a good smear on the rubber coupling that sits between the pan and tank, covering the nut ( you have got one haven't you!)
Pete K
January 2006
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