I think Thomas is right. Rather than use weights, you really should just put a bullet through the front of the tank. Since they weight plants in aquariums down with lead, obviously lead bullets don't pose a big threat so you should be okay there.
... or what about throwing a piece of live rock at it? That's ... like... a reef safe way of doing it.
Whatever you do, if it's a reef tank, don't use a bullet with copper in it.