Hi,
Easy, check the Aquarium Fert Dosing sub-forum and read this sticky:
http://ukaps.org/forum/viewtopic.php?t=13
It's exhaustive and it mostly targeted at highly lit tanks since
these tanks are more prone to nutrient deficiency.
A quick macro primer:
Nitrogen (N), Phosphorus(P) and Potassium(K) (as well as Carbon) are the macro nutrients and are what plants mostly need. The so-called "micro-nutrients" or trace elements as they are also known are found in the Flourish that you use. They are referred to as "micro" since plants only require very small quantities of these elements such as Iron, Manganese Copper etc. Plants really depend on the macro nutrients for survival. These 4 elements N P, K and C are the building blocks of plant physiology.
N - Nitrogen is the primary element of which chlorophyll is composed.
P - Phosphorus is a component in DNA and cell membrane and so is key in many growth functions.
K - Potassium is used in almost every plant function. It promotes photosynthesis which generates the formation of carbohydrates,oils, fats and proteins and then it also is used to transport these products to storage locations such as seeds and roots. Potassium also is used to enhance the efficiency of Nitrogen, enhance the absorption of water, and in the development of roots.
If you are not dosing these elements then the plants have to scrounge for whatever they can find dissolved in the water column (from tap water) or in the substrate. Nitrogen is the easiest to find probably because fish waste or decaying food generates NH3 (ammonia) so the plants can absorb and process this from either from soil or water. K and P are probably in the foods that you are feeding to the fish so this may also be available in small quantities. Because your lighting is reasonable the demand for these products isn't as high. As I was saying earlier though, as the plants grow they need more of everything so for a while you were actually dosing these but it's likely that the plant growth is starting to outstrip what you are feeding.
Macro nutrients are widely available. The basic choices are either to buy commercial products like Seachem Nitrogen, Potassium etc, or Tropica Plant Nutrition+ which is very good because it has macros and micros. The other option is to purchase the dry potassium nitrate KNO3 and potassium phosphate K2PO4 and dose these individually. A popular place to buy these online is AE;
http://www.aquaessentials.co.uk/
Have a look at these links and if you have any further questions just let us know.
Cheers,