Hi there! Sorry to come into this late

What have you been feeding your goldfish? They really need a mostly vegetable based diet - otherwise bloat eventually catches up with them. Having them graze on aquatic plants, such as Elodea/Anacharis or other tender plants, works great - here's more info on that:
http://wetwebmedia.com/FWSubWebIndex/gldfshmalnut.htmBut if you can't manage that, just feed them veggies. Apart from peas as Lyvea mentioned, mine eat chopped spinach, zucchini slivers, cucumber, slices of Romaine lettuce - which I just weigh down with a heavy magnetic clip (like a Chip Clip). I also feed them nori, which is the dried roasted seaweed you use for making sushi. I just tear a strip and weigh it down with a plastic clothespin. They love these foods, and I have never had them have trouble with bloat or constipation - their poop is always green and thick (good) not thready and white (bad).
Water quality could definitely be an issue with such old goldfish in a 29 gallon tank - they really do need more room! I have two black moors in a 29 gallon tank. Any minute now (I've been saying this for months now

) I am going to find the time to set up their 55 gallon tank. A 55 gallon tank can keep 3 goldfish, provided they are not comets, shubunkins or any of those long, koi shaped fish. A 29 gallon tank can technically keep two, but you need a great deal of filtration and water changes to keep things healthy. 40 gallons is much better - goldfish really do best with 20 gallons per fish, I've come to find out

Do try the peas, and if that doesn't fix matters, dissolve Epsom salt (magnesium sulfate) in warm water at a ratio of 1 teaspoon per 3 gallons. This works as a muscle relaxant and will relieve constipation, if that's what the problem is...but do check that water quality!