Clear stringy, slimy poop is a strong indicater of an intestinal infection caused by protazoans, the best treatment I have found is a mix of metronidazole, praziquantel, and something like levamisole. There are several products available, especially for discus breeders. One I have had good results with is the antiparasitical medicated food made by Jungle AP, google that for a source, I think BigAls.com sells it. The jungle antiparasite tank buddies can also work but not as well as the food.