Maybe it's a sign the fish are a little constipated? Do you feed them mostly commercial goldfish fish pellets or flakes? Trying giving them some vegetable matters such as boiled peas and see if it helps any. Too much plant fiber can cause diarrhea so you have to adjust their diet accordingly.

Stringy poop is a sign that something is wrong with your fish.  Healthy fish poo is short and the color of the food they eat, not long and stringy.  Your fish could be suffering from a bacterial infection or poor diet.

I actually read that fishpoo is supposed to have a rather brown color and crumbly texture. And that if the poo actually has the same color as the food you gave them, something is wrong in their digestive system... but of course, if they have a diet rich on vegetable and plant matter, then it's only natural that it's a bit more green than if they eat pelleted food and bloodworms, for instance.
But it's not supposed to be stringy! So I think you might be overfeeding them a little. Try reducing the amount of food, but increase the times of feeding per day from 3-5 times. (By the way, you shouldn't give them bloodworms more than twice a week as a maximum! Goldfish can get constipated from too much of proteine.) But do try the peas!

Ah yes, I have heard about many different methods of feeding the fish, and all of them are supposed to be the best o_O
But so far I trust the one with many feedings a day in small amount the most, since I have rather clear and good reasons why (: It's because goldfish can't store proteins or any kind of nutrition in their body for long time (they don't have any stomache) and therefore they need just a small amount of food a few times a day.
I read about some specialist who's feeding his goldfish in that way you mentioned too, like, as much as they can eat for a couple of minutes twice a day or how it was. But I dunno... maybe that's meant for bigger goldfish?

cough, cough..
thought i would share some of my findings on this interesting topic 

-medium to short chunky poop or poop that is the color the food = good sign - this is what you want
-long thin poop = not eating well, probably stress related
-long and thick = constipation or overfeeding - if overfeeding then the food is moving through the fish too fast without adequate time to be digested
-clear  = reabsorbed eggs
-dark and sausage like = gas trapped in floating foods, fish eating bubbles, foods being fermented by gas causing bacteria
no poo seen but the water is slightly brown - perhaps diarrhea that has blood - argg not a good sign 

It might be an idea to treat your other fish for ich too, in case they already were contaminated by these two! At least before you move them back to their home (:

Thank you, Seawitch.
When the spots are gone I put the fish in a treated clean water and dump the yucky water away to get rid of the ich and their possible eggs in it. The treated clean water was set to sit there for days until the fish are all clean and ready for it. I hope that doesn't cause any other additional problem. It worked before twice for me but then again it could have been beginner luck?

thanks, nosie. I treated them all just in case. Still recovering.
Katherine, ich is a bacteria infection. We just went off topic a bit from the poopstring discussion.

huh. very interesting. reason i asked is cuz im treating my common for ich. ANYWAY do peas clear up fecal issues in the fish?

true dat Bon Bon. :)I hate seeing fish sick. i hate having fish die too. well, everyone here seems to know what theyre doing so as long as we follow everyone elses advice we all should do okay