I too have 3 species of Metynnis, in a 75 gal planted tank (see below). My experience has been a bit different.
I maintain frogbit and duckweed in the tank (replenished every other week) and they like it.
At this time I have Java Fern (Microsorum), 2 different Cryptocoryne (tall one, green, and a short purplish one, probably balanseae), and another plant (tall, very leafy whose name I don't know). Fish have been in this tank for > 2 years. Everything in the tank is natural (not fake).
I started with plastic plants and over a few months changed fully to real plants.
Good luck!