Some questions about common plants

First of all hello and thanks for all the advice & info floating about
on the group. I have just started, and am getting interested in plants
in the tank - more interesting than the fish! I have a few no doubt
stupid newbie questions and would like to apologise in advance for my
lack of taxonomic specificity (!) or poor latin
I have what I believe is a hygrophilia polysperma (id in the petshop
by comparing with some plastic plants which were labelled - the live
ones were not). I bought this as I had read here that this was a very
fast growing plant but, not a sausage, no O2 bubbles, no discernible
growth. The plant is very brittle (lots of bits broke off when I
bought it), and is a grey/green colour. Is this normal?
I also have some ?elodea crispa, this seemed to grow quite well, then
stopped, then began to sprout side shoots of a completely different
colour, vivd light green as opposed to dark on the main stems. At
first I thought that these shoots were flowers, but they kept on
growing. Is this the emersed/submerged form difference I have read
about? Some of this plant has lost a lot of its old darker leaves and
roots have sprouted from the junction with the stem and new growth. Do
I cut the plant here, discard the rest and plant the root and new
growth or let nature take its course? The fish seemed to prefer to eat
the old growth to the new.
I also have what was described in the shop as cabomba. this grows very
well but the bottom portion goes brown is this the same as above?
(this seems to throw out roots in odd places as well) Also with the
cabomba some small strings with bobbles on them have been produced and
have detached from the plant, are these seeds?
I also have some "amozon sword plant" this did nothing for a while,
then small leaves started sprouting arond the bottom. they seem to
have stopped growing,and are spotted with brown. The original, larger
leaves have gone yellow (there were brown spots on them when I bought
I have a 15g tank 15" tall, gravel only bottom, no fertilisation, ugf
(air driven) CO2 from yeast bottle (I know it is said that surface
disruption negates the effect, but this definitley makes a difference
to the amount of O2 bubbles coming off of the plants) 36w ordinary
cool white bulb  and 18w Philips TLD/18w/95 on for 12/24,and about 9"
of fat, messy fish. Water changes about 20% every two or three weeks.
One last thing, I have read that vermiculite is sterile and inert, is
this so, and if it is, why do people use it, and how can it
help?Thanks very much for your time if you have got this far, and
thanks even more if you find time to reply to any of these q's

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