Hi,
pH alone is still inadequate. Could you please include your ammonia, nitrite and nitrate? What exactly are the results? Post the numbers as given to you by your test kit.
What brand of test kit do you use? This is also another important question as most test kits are grossly inaccurate and misleading. Some are also expired or tampered with that they are not reliable anymore. Test strips should be avoided as much as possible. API liquid drops are best advised for use if you can find this brand.
What do you feed your fish?
What is the temperature of your tank currently?
What is your filtration system?
How often do you perform water changes and how much water volume is replaced?
As far as white stringy poo is concerned, it may either be internal parasites or internal bacterial infection that is currently the issue. The trouble is internal parasites normally do not kill your fish quickly. They can linger for awhile and your fish will look like it is wasting away. Have your fish actually looked wasted away or skinny? The body should be rightly proportioned and not looked bony. The abdomen should not looked abnormally distended especially towards the anus or deflated. In this case, metronidazole, praziquantel, flubendazole or fenbendazole are your best choice of treatments. The whole tank must be treated.
For suspected internal bacterial infections, this is trickier to diagnose. Have you observed other symptoms? If so, what are they? Did the fish swim listlessly? Is it gasping to the surface? Are the fins clamped? Since it is not very easy to determine whether the bacteria being suspected is gram-positive or gram-negative, what we need is a broad spectrum antibiotic. A combination of Maracyn and Maracyn 2 in this case will do the trick. You do need to treat the whole tank. If possible, mix the combination of two Maracyns in their foods and add garlic to hit the internal bacterial infections directly by ingestion.
Have you observed any extreme outward aggression between the fish? When you found the wound on one's mouth, did you even isolate the fish imediately and treat it? Do you have a shot of the mouth? How severe is the damage? If you did not treat the wound, chances are good you are letting bacterial infection set in. Mouth is one of the most critical areas aside from the gills. If not treated immediately, secondary infections set in and can kill your fish almost immediately.
