While Erythromycin is a good and effective antibiotic when used correctly it is primarily best used against gram-positive bacteria. Unfortunately most bacterial infections that effect our fish are gram-negative. Methylene blue is also a very effect treatment for fungal and some parasitic afflictions however it is very hard on nitrifying bacteria and I usually only recommend it’s use as a bath or in a hospital tank and avoid it in main display tanks for this reason. To me this looks like HITH (hexamita), I would treat this with Metronidazole. If you can’t find straight Metro use API General Cure (Metro/Prazi) in combination. Since this fish is in a hospital tank do a 75% water change, add 1 tablespoon of salt per 5 gallons of water and treat with the Metro as per instructions. In addition dissolve a 10 gallon dose in just enough water to dissolve the powder and soak some pellets for about 15 mins. Dump the whole into the tank and let the fish eat it. Going forward keep the water very clean and make sure the nitrates in the main tank say below 20ppm lower is better when it comes to nitrates so filter maintenance and substraight vacuuming is key to maintain good water quality. Also avoid live foods a good quality pellet is the best diet. If you have any questions or concerns please ask. HTH.
