Flourish essentially contains zero nitrogen so will do nothing for the nitrate levels of your water. Very few aquarium fertilisers have nitrogen or phosphate because they are considered to promote algae growth and normally fish waste will provide enough of these essential nutrients.
Some fertilisers do have nitrogen and phosphate. NA
Thrive if its available in your location (N. America). We have TNC Complete here in the UK. Aquarium Coop do a fertiliser that has these nutrients in them, Tropica too.
Before we go down that route, can you tell us a little more about your tank. How big is it? What fish? How many? What plants?
Most aquariums stocked with fish and planted with low demand plants will not have issues with low nitrate. I would double check you are doing the nitrate test correctly. Really shake the heck out of bottle #2. Like bang it on the countertop. Take a sample of water to your LFS
and get a 2nd opinion.
You might be over lighting too. 12 hours is a lot, and it could be causing the plants to strip out all the nutrients and causing poor growth.