Blyxa japonica
Difficulty: Easy
Flowers are tiny and inconspicuous, and could easily be mistaken for some kind of runner.
I noticed it's very easy to get this plant to produce flower spikes under strong lighting, but the flowers only seem to 'bloom' if they're able to reach above the surface or are emersed for long enough.
This particular one had flower spikes for a long time but only bloomed when I put it into a freezer bag for a day to give away.