I turn off the pumps, mix up a THICK paste with kalk and water, then use a little pipette to smother them. Getting a chunk of it in the nem's mouth before it realizes it's not food is a bonus! The paste sticks to the rockwork even after I turn the pumps back on; I leave it in there, it doesn't seem to hurt anything and dissolves after a day or so with no Aiptasia in sight.