This symptom (stringy poop) can have two different causes. First, what color is the poop? If it's dark, then the fish may be constipated. Try feeding some peas to help clear the constipation. I feed my bettas peas once a week as a preventative. They gobble up the peas with excitement!
Boil some frozen peas (canned ones can get too mushy) for 4 1/2 minutes in the microwave. Tap water is fine to boil them in. When they're done, pour off the boiling water and add cold water, pouring it off and replacing it a few times until the peas aren't hot anymore. Remove the skin and throw it away. Cut the pea into small pieces with a paring knife (yes, it's a bit tedious!). You want to cut the peas into pieces that are about the same size as the food they eat every day, so they will accept the peas. Feed the peas for one or two days in a row. Then don't feed anything on the next day. Also, if your fish like veggies, you could boil a zucchini enough to soften it and put it in the tank for the fish to nibble at once a week. These veggies will add some fiber into their diet and hopefully will help clear the constipation.
If the poop is white and stringy, the fish may have an internal parasite. Their stomach area may look sunken in too. If this is the case, try the Jungle brand Anti-Parasite food. The pellets are rather big and hard, and you may want to soften them in tank water and cut them in half before feeding them to your fish.