I wouldn't know precisely how to quantify how many more plants, (lots are
good...) but in addition to the fast growers (the so-called oxygenators of
pond fame), you might want to consider emersed plants. I have had a potted
plant (Spaths or Spathiphyllum) grow out of its tank and send runners to
other aquariums, ultimately colonizing about 15 of them. Other tanks have
been started from divided plants. Many starts have been given away, often
being first placed in outside filter boxes. Temple plants also will grow
emersed rapidly too, but need more light. (You can burn the Spaths in
sunlight - they do indeed need diffused light, as do several of our house
plants originally from under the tropical forest canopy.) They have
impressive root masses too - somewhat reminiscent of the stream bank photo
in Allen's more recent Rainbow book (p. 169) where all of the grass roots
form a mass reaching over the stream.
Not long after that first Spath climbed out of its tank and began to devour
Park Forest (our town), Diana Walstad published an article in FAMA
(Freshwater and Marine Aquarium) Magazine touting the virtues of cultivating
plants which grew out of the tank. Her contention was that plant leaves in
the air or floating on the surface could more efficiently exchange CO2 and
O2, photosynthesize ( say ... ten times more efficiently than submerged
leaves) and do all of the plant things we would encourage at dramatically
increased rates. (I believe she is also active on the Aquatic Gardeners'
List too, certainly has been involved with that group.)
Not really interested in high tech nor in labor intensive tanks, her 1999
book summarizes a number of her ideas. "Ecology of the Planted Aquarium, A
Practical Manual and Scientific Treatise for the Home Aquarist" was not the
easiest read this last summer. Those into chemistry will revel in it. She
does challenge the fishkeeper to look more carefully at why they keep
plants, what they are actually doing to encourage growth and what they can
do to raise healthier fishes in plant laden aquaria.
Water sprite, water lettuce, water hyacinth, the often annoying Salvinia,
duckweed and Wolffia and a variety of emersed plants may serve as something
of a freshwater variant of the marine "algae scrubber" for modestly
populated tanks. For those of us, busy to distraction, these tanks, with
lights on timers, are a God-sent.
All the best!