It’s a 180, but it’s not much work. Just 50% WC every other day that I use to water my lawn . 20ppm was before my last WC20PPM nitrate isn't ideal, but I very much doubt that it is affecting an aro to any extent, especially over such a short period of time. You will be working hard changing water to reduce that level or even maintain it in a tank that size (150 gallon?) containing that fish.
I would have moved that fish on to a larger tank long before it got that big, rather than moving it in. You are lucky that it wasn"t the 24 inch specimen you were talking about in your other thread...but it will be soon...