you should not have ANY Nitrite.
perhaps you are experiencing some cycle.
you should however have some Nitrate (say more then 10 ppm
but less then 30 ppm
) preferably closer to 10 then 30.
if you can test for iron do so as the plants need some of that, also make sure you have some phosphates. potassium i do not think you can test for.
how old are the light bulbs? you said the tank has been running for 4 years, if the bulb is that old it may have lost enough to not be much good to the plants anymore (fluorescent bulbs should be changed about once a year)
there may be some kind of contamination, but that would affect fish too
put in brand new bulbs (something in the 6500 Kelvin to 10,000 kelvin range)
check nitrate, Phosphate and iron levels post those here. (you may need to buy ferts and dose accordingly, based on test results)