One thing that may help is adding some floating, fast growing plants. Hornwort is an excellent plant to soak up those nitrates. When I started my tank I also had problems with Nitrates no matter how many water changes I performed. I added a bunch of floating plants, which sucked up those Nitrates like a sponge. The tank has been up and running about two years now and I have no problems. I have removed most of the floating plants, but I always keep a small bunch just in case. They grow very quickly so you will always have to thin them out or they will take over and shade the other plants.
Sent from my iPhone using Tapatalk