Nitrates are fine at this level and are good for your plants.
If you feel that you must get them down there are 2 ways I think are the right approach for this kind of tank.
1 is more water changes more often. That can be a lot of work.
2 is to use something like this. Costs a little money and takes a few weeks to a few months to get established but after that requires no more action on your part.
Seachem De-Nitrate Filter Media at PETCO
It's just pumice. Bacteria live inside and get rid of the nitrates by turning them back into nitrogen gas. The flow rate has to be right. If your concerned about that just use Seachem Matrix instead. Same thing, bigger size pumice. Flow rate is less important.
In all seriousness though...I think you should stay with what you already have.