I can't say I am surprised the clay 'taco' failed. While some clay in the soil is desirable, according to Charles O. Masters is his book, 'Encyclopedia of the Water-Lily', a substate too heavy with clay binds the nutrients too tightly; to the extent they become unavailable to the plants.
I also think you will find photoperiod is not a factor for tropical waterlilies. The Anecphya/Brachyceras have evolved over the eons in essentially equatorial zones, where the days/night periods are fairly evenly split. Hardy waterlilies are more temperate/boreal and they have adapted to shortening days and undergo physiological changes that make the rhizome more able to survive colder temperatures. For tropical Nymphaea the trigger for dormancy is primarily drought....the tubers being an adaptation to survive the dry periods.
Again, to cite Masters....temperature is the more important factor in the tropical Nymphaea. And one reason in a litany of reasons that correct names are so important in dealing with waterlilies.....in a private conversation I enjoyed with Dr. Barre Hellquist at an IWGS symposium.....in his on site observations of the Aussie Nymphs, temperature actually becomes an ecological niche. N. gigantea is the first to emerge when the rains return....as the temperature increases, the gigantea growth suffers and N. immutability becomes the dominant species; but, as the temps approach 40 C ( 104 F ), it is N violacea that attains dominance. So the sellers that peddle N. 'Albert de Lestang' as every Aussie lily in the book are being deceptive and that lily ( which is actually now thought to be N. carpentaria, according to Carlos Magdelena of Kew Gardens, will not perform as promised as the temperatures change.
Long way of saying, if you are growing seedlings, it is critical to know exactly what it is you are growing. Always ask for provable provenance. If I have time this week, I will try to provide some pictures of how I deal with seedlings.
Truth is not what you want it to be; it is what it is. And you must bend to its power or live a lie.”― Miyamoto Musashi
"Knowledge speaks, but wisdom listens." ~ Jimi Hendrix
“The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.”― Stephen Hawking
Craig