I suggest Jasmine Brough's article at
http://animal-world.com/encyclo/fres...ntProfile1.htm
A lot of sound info there.

The number one reason for poor plant growth is inadequate light -
most need a lot of it, like 100 lumens per litre of tank
capacity.

The next common problem is the planting medium - epoxy-coated
pebbles are of no use to plants. Many plants don't like
undergravel filters either.

Additional CO2 is useful only when plants are growing vigorously
enough to use up what is naturally there. Except for unusually
soft water which lacks micronutrients, the same is true of
fertilisers.

Good luck!