I have had aphids on all the duckweeds, Lemna and Spirodela. I've never been able to keep Azolla alive long enough for the aphids to try it. The only aphid-free floating plant I can think of is water hyacinth -- however, I keep my water hyacinth in a non-fish container so that I can spray them with Safer's Soap to prevent spider mites! Dean Sliger Deansliger at aol_com