I would start with a water change to help reduce the level. Make sure you aren't over feeding as well. Assuming that your tank has already cycled being that it is 6 months old, you can buy some Prime to detoxify the ammonia. I have used it my times in the past. It will not remove it but it will make it less toxic to the fish. Regular water changes ( we do weekly) will help. What kind of filtration do you have?
Is this the only fish in the tank? If not are any others acting odd? What kind of behaviors are you noticing with the bangai? Lethargy, heavy breathing, excessive hiding, swimming into the flow of a powerhead/filter, any odd spots patches etc.? How long has the fish been on the bottom like that and how long have you had it? What and when was the last wet thing added? As stated above you do have some ammonia readings which may be what is affecting the fish, however I also want to rule out any symptoms of disease. Definitely get on the water change and also try to find the source of that ammonia spike.