I agree with Scrivens, 50% weekly water change would be a good way to go
Twice a week for now until the levels sort themselves would help the fish too.
Nitrates ideally need to be less than 40ppm, so keep that in mind for the future. Keep an eye on the ammonia as well
In terms of vegetables, my goldfish loves to eat peas, brocolli, carrots, spinach, all of which I blanch quickly in a microwave (you could also boil it in a pan if you prefer).
My goldfish also loves to eat my Elodea Densa plant, so if you ever wanted to introduce some plants they could munch on, that would be a good one to try. Although be warned they might eat through it very quickly