Are you sure the nitra
tes are 0 and not the nitri
I would suggest something as quick and easy as a large water change
How much do you usually change and how often? Do you clean the gravel with a gravel cleaner when you do maintenance?
From now on I'd recommend 50% minimum a week. And you could feed them twice a day actually
They'll grow better and bigger with more nutrition since goldfish can't store it
So, now test the water again, write down the results (post them if you want) and change up to 70% of the water.