New set up

Hi, I'm new to this list so if I ask something stupid I apologise in advance.

I have been struggling with keeping plants in my 4 ft aquarium since I
moved house nearly 5 years ago.   The aquarium lives away from any natural
light.   After stripping the tank down again last weekend and buying
another 10 plants I read the FAQs.  I only had 1 old fluro tube and decided
that must be my problem.  This weekend after seeing all my new plants
looking fairly sick, I bought a new double light, and two new fluro gro
tubes 36 w each.   After searching the FAQs again I also bought more
gravel, some new tubing and fittings and set up a CO2 bottle which is now
trickling CO2 into one corner thru'  an airstone.   I switched off the air
pump during the day to stop the CO2 dissapating straight into the air.
However, at night I switch it back on.

I have a few questions though:

How long should it be before I see a change in the existing plants?

I have an underground filter.  Is it enough to switch it off during the
day, or should I disconnect it altogether and try something different?  Any
suggestions what?

I know nothing about adding iron.  Would an aquarium carry it or would I
have to purchase it from a general nursery?  What sort in Australia and how

Is there any need to measure the amount of air being emitted by the
yeast/sugar mixture?
Is there any problem with my four gouramis and angel standing in the corner
and sucking on the C02 bubbles (they remind me of a group at a party
sucking on a joint :-))

I have never bought any kits to measure Ph or nitrates, but I am sick of
these dying plants, I want to get serious now.  Do you think I should buy a
couple of kits?

Thanks a lot for listening and for any answers.