Fastest Growing Plants?

The species of Hygrophila (corymbosa, difformis and polysperma) are all really fast growers and very reliable.

If you are going to be an ichthyologist, you need to start using scientific names 

The problem with common names is that different places use different names for the same thing, and the same name for different things. Most of the members on this forum are from the UK. You mentioned petsmart in another post, so I assume you are American? We likely use different common names from you which could make it a bit tricky deciding what is what.
For instance, I would call the plant you name elodea not ancharis. I had to google ancharis to find out what it was. Other people would call this plant egeria.

The plants Richard named:

Hygrophila corymbosa = temple plant, staghorn or giant hygro
Hygrophila difformis = water wisteria
Hygrophila polysperma = dwarf hygrophila, dwarf hygro, Miramar weed, or Indian waterweed.


If you call the last one dwarf hygro and I called it miramar weed, we wouldn't know we were talking about the same plant!