Maybe you should try not feeding him for a while. Just for a day or two and see if it helps.
A long string of excrement is a sign that the fish ate too much during a short period, and if your fish have a bloated appearance, they might just be a bit constipated. So you could try feeding them less, but many times during the day! It's very easy for a goldfish to get problems in his digestive tract since they can't store any food in their stomach.
You could try feeding them shelled peas after these few days and see if they get better (: The peas should clear out the digestive system of your fish!
And if it helps, I could list a few things that may be the cause of your fish sitting at the bottom:
- Water quality (which you checked already)
- Parasites, try looking for symptoms of these! Anything strange on the skin of the fish? Is there anything on their gills? Is there too much mucus on the fish's body? Is your fish rubbing himself on the tank sides/decor? Look for anything unusual!
- Swim bladder. This could be a symptom connected to the fish being constipated, so the peas and the few days without food should work fine!
If you want to, you could try adding some salt to the tank, it relieves the osmotic pressure on the fish's gills for one thing
I hope your fish will be alright! (: