Some tetras are rightly described as aquatic "lawn mowers" in that they will eat just about anything green, Silver Dollars, Bunoa Aires, etc. Quite a long list. The small tetras like Neons, Cardinals, Phantoms, etc., are not so inclined though.
You say red and green striped plant... that makes me wonder. Are you sure the plant is an aquatic plant and not a terrestrial plant that is simply rotting because it is underwater? Most true aquatics with red colouration require very high light levels and will not survive in low light environments, could also be a clue. A rotting plant will be eaten by a plec., although most plecs are not that bad at eatng healthy plants, not the smaller plec's anyway.