Sand is the best substrate for goldfish. IME though many sands are not good options. Not all sands are equal. I only use and recommend Estes' Ultra Reef. It is NOT a true calcium based marine sand. But it comes in multiple colors, is extremely uniform in size (the perfect size of sand IMO
), and super easy to keep clean.
I assume you have tried to express the issue of more than two or three fancy goldfish in a 29, not to mention 6 med/lg goldfish that may be pond types.