How do you cure goldfish with white poo?

It may be stress or it may be a sign of something more serious. Keep the lights off, keep the water healthy and offer them only plain cooked veggies for the next 4 days.

Broccoli florets (flower part) is a good one that goldies love. If its just stress, you should notice his waste as nice, firm healthy strands of green. Do not offer any fish food. Just lots & lots of veggies. You can also offer some brine shrimp (defrosted not freeze dried) once a day for some protien. It also has a laxative effect. Watch him closely & keep us posted on how things look.