The only thing that would make your light too bright is algae growth. Other than that, having very high light intensity just drives the plants to grow faster. You say you add "some liquid fertilizer" once a month. No matter what the liquid fertilizer is, you are not adding it nearly often enough.
All plants, whether terrestrial or aquatic, need nitrogen, potassium and phosphorous, plus a mix of "trace elements" like iron, boron, etc. in order to grow. These elements must be added as fertilizer. Plants also all need carbon in abundance, since plants are made up primarily of carbon. We usually add CO2 as a source of that carbon, but Seachem Excel is another such source, and in low light tanks, the substrate can provide enough carbon to support the very slow growth those tanks have.
You need to do some research on fertilizing and on keeping planted tanks in general. This,
http://www.barrreport.com/you-new-aquatic-plants-start-here/3009-newbies-please-read-1st.html, is a good place to start, plus reading some of the other threads in that forum.