Go to the illness forum and check out the stickies for bloat. Therre is a clout cure and a metronidazole cure. I have used clout with success once the fish show symptoms, and the metronidazole on the show tank to nip any developing cases in the bud.
There are two links regarding treatment of bloat below my signature.
Let's get some more information (be as detailed as you can - don't leave anything out) and see what you need to do. Meanwhile, you can add epsom salt to the tank at 1 cup per 100G preventatively for those that aren't affected.
If the only problem you're having is white stringy feces, I wouldn't jump in and treat for bloat. Fish can have light coloured feces for many different reasons.
Should any of them begin to refuse food, spit food, or behave unusually, then you may need to treat.
The epsom salt is a good preventative. You might also feed some medicated antiparasitic food for a few days preventatively, as well. Jungle makes a pellet that contains medication, or you can add some metro to the food you feed before feeding.