LECA (not leca) means Light Expanded Clay Aggregate is the same material you mention, Peter. It is very highly water absorbent and these conditions are the opposite than for adequate gas exchange mixes (like those for bulbs and succulents). I am anxious about your Polianthes geminiflora, a very scarce species in cultivation, under this hydroponic like method. > I have never heard of leca stones but they look like what is available in > the UK as "Hortag" which are hard expanded clay pellets and make a good > moist substrate for gravel trays. I experimented with those in potting mixes > for a while with some success, but had a disaster with a filmy fern because > they are aparently alkaline.