Best Plant for Guppies
Back in the "dark ages" and I'm talking about the late 1950s and early 60s the plant of choice for Guppy tanks was Water Sprite also called Water fern and Indian fern.
The "water keepers" of that day didn't have anything fancy and would judge the condition of the water by the condition of the Sprite. If the Sprite thrived, then the water conditions were right and the Guppies thrived too.
I have several large tanks of Fancy Guppies, probably a 1,000 individuals or more (how would one tell anyway) and have floated Water Sprite in all my tanks. The fry survival rate in my tanks is quite high, maybe because of the Sprite.
Get the Sprite in the tank and you'll have a lot of Guppies.
"Fear not, my young apprentice. Just change the tank water."