Oh yes, I have dealt with this type of decease on numerous accounts, but not with aros. Fish carry varous forms of internal parasites, which remain dorment until environmental conditions activates them. 'Stress' is the main contributor; cooler water temps, competing for terrority and food, high traffic areas around the tank, poor water chemistry.
Being an owner of several types of exotic aros.... I suggest that you use FURAN2, it works the best for me!
What size type is it housed in?
What tankmates are/is housed with it? [seperate the RTG from them]
* Your pH is fine.
* Increase water temp to 86 degrees
* Increase oxygen; air stone, difuser, etc
* If treating with others meds, do a 25% water change [aged water preferred] and add carbon for a min of 4hrs to remove the meds, then treat with FURAN2.
* NO MORE LIVE FOODS!!!, unless crickets, meal worms, superworms.
NOTE: 'Aged Water' - I use a 55gal storage container filled with tap water, add water conditioner [Prime], air stone, and heater... for a min 4hrs before adding Discus Buffer to adjust pH, then 1hr later, use it as 'Replacement water'.
Your pbass may have adjusted to the use of adding straight tap water, BUT your RTG will suffer from it, especially any amount of water replacement over 10% and if he is less then 12 inches in length.
Adding salt is not a bad idea at this point, but be sure not to over do it.... more stress on the RTG.
I hope this helps! Good Luck!