May i know how many bubbles per second is your CO2? As most of the Ludwigia we get are emerged form so it requires plenty of CO2 to convert itself into submerge form, why don't you increase your bubble rate and see how?
Even i haven't try the Ludwigia on my Nisso Volca Sand, i believe most demanding plants will thrive on soil-like substrate. Please wait for my good news.
My thread on Nisso Volca sand.
http://www.aquaticquotient.com/phpbb2/viewtopic.php?t=6331&sessionID={57449917-3918-4F81-B8CD-20843B9676A8}
Aquatic plants and LEGO collector.
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