Nitrates are steady at around 40ppm. I've used some nitrasorb stuff that brought it down to 15ish but it went back up to 40 n a couple of days. Goldfish eat to much if I don't feed it enough it starts to look sad.
When I had nitrates at 40ppm I got string algae. You probably need a stronger filter or do more water changes until you rehome the goldfish. Their like cows eat and poop constantly. They also love to eat plants.