The reason I asked about darkening of the tongue is that Mycotoxins have been known to cause that. Copper sulfate is commonly used at 1/4 tsp per gallon to treat fungal problems. You said the tongue was yellow, and I'm wondering if it could be advanced Canker. Is there any yellow plaque like substance in the mouth or near the back of the throat? If so, 250mg flagyl tabs (metronidazole) can be given 2x a day for 5-7 days for a 4+ lb bird, half that amount for a smaller bird. The capsules are also available in some pet stores under the brand name Fish-Zole.
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