How long have you had him? Repashy is great food for fancies but, some fancies do not do well on it. Does he get veggies?
I would start with water changes every other day to daily until you have an idea of what your parameters are. Put him of a veggie fast for a few days along with lots of water changes and lets see if there is any improvement. Feed him cooked, deshelled peas that have been smooshed along with spinach/other dark green and purple greens. He can have a few small meals of fresh or defrosted brine shrimp, too, which have a laxative effect. You should see big, firm strands of green waste with this diet.
If he improves, then I would start adding very small meals of Reoashy back into his diet along with daily veggies. Feeding smaller but more frequent meals (4-6x day) instead of 1 or 2 big meals will be much easier on his digestive tract and more closely mimic a goldfish's natural feeding habits.
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