Sounds like Bloat, symptoms are stringy white fecies, reclusive behavior, clamped fins, labored breathing, spitting or not eating food. Normally kills fish within 7-10 days.
I use Parasite Clear by Jungle which has never let me down whentreating for Bloat, or Tetras new medication has the similar active ingredients but I don't have any experience with that one.
After treatment the cause must be corrected wether it's incorrect diet(most common), stress, or a high level of salt. Stick with a high quality pellet containing under 38% protein. I recommend New Life Spectrum Thera+A which contains a high dosage of garlic which can prevent some parasites.