See section 3 of the following link for treating fish for intestinal worms.
Praziquantel treats tapeworm.
Levamisole treats thread/ round worm.
Flubendazole does both types of worm (flat & round).
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
www.fishforums.net
The fish looks skinny in the rear half of the body (between the stomach and tail). Plus the tail is drooping and the fish doesn't look happy.
Besides prawn make sure they get some plant matter. Goldfish need plants in their diet. You can use aquarium plants, Duckweed (a small floating plant), peas, pumpkin, zucchini, spinach, etc.
Make sure they are clean and free of chemicals. Peel them if possible and boil for a minute or so to soften them up.