It takes some patience, but get a medium gauge syringe from a pharmacy, a plastic cup, RODI and a tablespoon of kalk. Mix the kalk, heat in microwave to just before boiling.
Then push into the mouth of the aiptasia. It will kill them instantly, not hurt anything in your tank, and won't kill other animals (the nudibranchs die when aiptasia gone, or before). Sometimes (when you miss the mouth) you need to do a repeat on some aiptasia, but normally it kills them first try. For a tiny fraction of the cost.
Just saying...