Problem with constipation in Common Goldfish

Question
David St hi. We've had a long term problem with one of our goldfish for about 2 years now. He is a Common and lives in a 300liter tank along with 3 other goldfish (Comet, Black Moor, Shubukin). We do weekly water changes (about 25%) and all tests show that the quality of the water is ideal. We have 2 water filters,the internal Juwel that came with the tank and an external Eheim, Small pebbles as the floor and 3 rocks that are suitable for fresh water aquariums. Their main diet is mostly Ocean Nutrition Goldfish Formula Flakes, Ocean Nutrition Formula2 Spirullina Flakes and Pellets, Wardley Goldfish Ten pellets, Tropical Goldfish flakes, Hikari Flake Goldfish, Hikari Chappy pellets, and Japan Pet Yamato Gold pellets.
We also add 1tbl aquarium salt (API) for every 40 liters with our weekly water change as well as the recommended dosage of API stress coat. We feed them twice a day. The water temperature is controlled and is at a steady 24C.  We also have a Tetra 400 UV lamp. Now to our problem.  Our Common goldfish named Chippy for a very long time now goes through fazes of constapation. For the last 8 months we thought we cured it but unfortunately the problem recently reappeared. For the last 15 days all for fish showed signs of gastro problems. Their feces were white and thread-like.  The other 3 seemed normal and energetic but Chippy stays in his corner with fins and dorsal close to body and doesn't eat normally. Also when trying to go he scrapes his bottom fins on the pebbles and sometimes dashes from one end of the tank to the other.  We tried to resolve the problem with extra salt, peas and fasting but no change.  Because Chippy was not showing any signs of rehab, we added Sera Baktopour Direct and repeated the treatment(with UV lamp off and no carbon filter). The other 3 seemed to be cured as well as Chippy, but after 3 days Chippy's symptoms showed up again.  We then went to our local aquarium shop and told them Chippy's problem and they gave us Aqua Furan. We put Chippy in the hospital tank (60 liters) and gave him the therapy witch lasted 5 days (first 3 days no food). They told us that Aqua Furan is one of the most effective and powerful medicines for these types of problems. After the treatment we put Chippy back in the main tank.  He seems more energetic now but when he eats he goes back to his corner and seems constapated and eventually does white thinner thread feces but with much concentration. Thank you in advance.  I sincerely hope you can help us because Chippy's been going through these fazes for about 2 years.

Answer
Hello:  I would try to feed chippy live or frozen brineshrimp.  The shrimp have tiny little shells on them that act like roughage and makes it easier to go to the bathroom.  You can also try an internal parasite medication that is eaten as opposed to treated via the tank water.  The list of food that you have given is all very dry... especially the pellets.  Try to vary their diets with blood worms, brineshimp, frozen vegetable cubes, etc.  I think the pellets take a lot more work to digest then if they just at a little bit of many different types of food... Those are the two suggestions that I have... keep me posted... dave