| | Re: Hygrophila sp 'Brown'?
Step one to flowering would be emersing it, especially if it tends to be low-growing or creeping as the stem in your photo appears. If it's taller, allowing it to grow up out of the water would potentially work. Hard to say what would trigger flowering without knowing at least the genus (catch 22 much?). Growing a bunch of stems and trying different conditions is the safest bet.