It's not a common or cheap plant. I'll tell you that much. It used to be sold by a shrimp dealer on this site....for around 30 something a head. And do realize each head is about a quarter in size lol. Also idk why they would suggest it's for reducing ammonia. I think they meant plants in general....
id reccomend just about every genus of plant before eriocaulon for that.
all plants will absorb NH3/NH4+, and the faster they grow, the more they will absorb.
get some egeria densa at your local petco. or find some hornwort, or any other floating plant.