Congrats on the spawn and on them taking to a tile
I had the same trouble with the nanochloropsis culture that I bought in bulk. I had better luck with a culture disk:
https://3kserver7.com/~frank/secure/...=LIVE_CULTURES
Several people have mentioned too, that it's a whole lot easier and less mess to just use the algae paste, so that is what my main food source for my rotifer culture is.
Also, be sure and check out the links on the stickied thread at the top of this forum.