Your lighting sounds suffient, plus they get filtered light from your window. FW plants need the right kinds of light. Natural daylight spectrum, like a 15 watt strip light is enough depending on the depth of your tankl, or specific plant gro bulbs. keep your bulbs changed out, just like you would on SW. aquatic FW plants do use elements found in tap water, without these elements they are robbed and will not grow. Here are what plants need to survive in a tank:
Beyond the "building blocks of life" provided by water and CO2 (oxygen, hydrogen and carbon), two other important nutrients are required: nitrogen and potassium. Nitrogen is usually available in sufficient quantities from fish waste in the form of ammonium (NH4+). Most plants will prefer ammonium but some will use the end product of the nitrification cycle, nitrate (NO3-). Ammonium is the preferred source since it takes less energy to use that form of nitrogen. A good test for ammonium levels is to monitor nitrates. If the nitrates are 0 ppm, you know that all the nitrogen is being used. This may indicate that some plants are starving for nitrogen. It also might indicate that a perfect balance has been achieved, but that is unlikely.
Potassium (K+) is also usually available from fish food. Unfortunately, potassium is difficult to measure in the water. If there are enough nitrates, there is usually enough potassium.
TRACE ELEMENTS
Trace elements are those things required in very small quantities yet are still vital to plant growth. These are taken in by the plant in ion form. The more important trace elements are sulfur (SO4--), calcium (Ca++), phosphorus (HPO4--/H2PO4-), magnesium (Mg++) and iron (Fe++).
Sulfur, calcium and magnesium are usually found in tap water. If the water has too little general hardness (< 3 degrees dH), calcium and/or magnesium may be in short supply. This can be remedied by adding calcium and magnesium sulfate in small quantities.
Phosphorus can be measured in the water and should be present in quantities less than 0.2 ppm of phosphate. If the nitrates are OK, phosphorus levels are usually also OK.
Iron may be present in tap water in the correct ionic state (Fe++) but will quickly oxidize to a form unusable by plants. To prevent this, chelated iron mixtures can be used. The chelator prevents the iron from oxidizing and makes it easy for the plants to assimilate. The iron concentration should be less than 0.2 ppm.
Other trace elements are needed in extremely small quantities and can usually be provided in fish food. Hope this helps