White stringy feces can be a sign of internal parasites, as can wasting away, flashing, listlessness, and refusing or spitting out food.
I treated my fish with PraziPro (a praziquantel med by Hikari) mixed with frozen mysis shrimp for a week or so, and then with Metronidazole mixed with the shrimp for a week or so. Also put some PraziPro in the water column. Other options are to use Panacur (a horse dewormer), Levamisole, or an already medicated food (Jungle Labs makes one.) Panacur has a broader range of parasites it will treat (such as camallanus), but is harder to readily find than praziquantel. These meds, BTW, should not impact your biofilter, and are not harmful to invertebrates.
I'm not an expert, though, so I'd recommend doing some searching and reading on fish illness to confirm signs and symptoms, and rule out all possibilities.