Hi! You should not wedge it but tie it to the wood with a sewing thread (it will rot away after some time or a nylon thread used for fishing). Plus if you do that I suggest you to cut its roots and leave them about 3 cm / 1 inch long.Anyway, it's clearly a rhizome plant so I've wedged it in to some wood as shown.
About your nitrates there's no need to keep them so high, probably the test is exaggerated but even if they were at 50 ppm I would bring them at 30 ppm at least. I have read that your tap water has already enough nitrates, so don't dose them if you can.
To me it's more about Iron and micros, but you can ask in Plant Help to receive more opinions. If you have that high KH and are using an airstone, it could be that the pH is quite high. You could try dose more micros and/or use a fertilizer which has the iron chelated with DTPA, which is a stronger chelating agent than EDTA. Don't use Iron gluconate, it will precipitate almost instantly and will not be available for plants.