Like yours - my readings for ammonia nad nitrite have always been spot on. But nitrate is a different story...
If I do a partial water change it does not alleviate the sitiuation much... And that's because nitrate level in my tap water is above 50... What can we do? Probably let the filter disperse it a bit and hope that as long as there is no ammonia nad nitrite in the water, nitrate itself should not cause serious problems. Plants and algae like it though, so they should be thriving... It's their food. And as far as I know goldfish tolerate it up to 100. Fingers crossed!