Cant speak on the lords, but my echinata has expelled its filimentary innards almost as far as an inch and a half. And when those things land, look out, can pretty much disolve a lot of different kinds of corals. My buddy was just tellin me how one of his friends got there micromussa zapped by his acananthastrea ech. 2+" away. So at least 4-5" for good measure plus take note where the current is and make sure nothings downstream from them that may be too close as that area will have its full extension capability.
-Justin