Most true tropical lilies such as the N. gardneriana and the N. glandulifera require high temperatures and lots of light to thrive. They are if you find them available in the trade rather difficult to grow and maintain. Ask
@Edvet i believe he bought a small N. gardneriana at the local aquarium exhibition in 2018, after previously failing a few times with the N. glandulifera.
I also failed to grow the N. glandulifera several times.
You can also find several tutorials online on how to go about with germinating nymphaea from seed. Some sellers at Ebay etc. offer Nymphaea seeds. This might be an option to try to germinate a number of baby plants and go from there. I've been playing with the idea to try this myself for some time now, but yet didn't get to it.
Which species of the lily to grow from seeds isn't that important, the majority is not really aquarium suitable anyway as mature specimen. But that isn't really problematic because in the indoor conditions we can give it will take a very long time to get to this unsuitable stage.
If you dive into the world of water lily you find depending on the species they grow different types of rhizomes (Tubers).
Mexicana for the tropicals (Bulbs) growing vertical
Marliac and Tuberosa for the hardy ones growing horizontal.
Most pond lilies available are marliac or tuberosa rhizome, these tubers are much easier to grow and can be compared with a Potato. They can be divided and grow on as separate plant. Now if you take a rather mature specimen of a smaller manageable pond lily e.g. not necessary but the ones labelled with Pygmaea in the name. Grown to sufficient size than (regularly) inspect the rhizome. You'll see growing eyes on it sprouting new young plants from it. These you can cut off replant and grow on.
Like this. The roots growing from the cut is about 6 weeks after it was cut.
Since the size of the actual plant the lily is very depended on the size of its tuber it grows from. A size like this will stay a dwarf for many years. The sp. from the cut off growing eye in picture above is a cultivar Nymphaea Burgundy Princess. Far too big for aquarium in mature size. But i'm growing it in a 110-litre low energy tank now for already 4 years it still is in a manageable size. And it took 3 years before it developed a new runner. I'm also still growing this lily in my garden but haven't dug it out and haven't cut it again in the last years.
To get to a carpet with a number of small ones like this obviously requires a number of cuttings. It requires some space and time to get it from other mature specimens. Depending on how many you want and the size of the mature tuber
It might be a multi-year project, but it definitively can be done with much easier to grow lilies than most available tropicals from the LFS.