There could be many reasons for the fish exhibiting this behaviour. He could have either injured his mouth or perhaps has something stuck inside.
Has anything changed in your tank recently ? (i.e. water parameters, addition of livestock). Have you noticed any erratic behaviour from the fish other than swimming with his mouth open ? Do you have any agressive/semi-agressive corals in your tank that might have stung the fish ?
Sorry, I realise I've given you more questions than answers. Perhaps review some of the above possible causes and keep an eye to see if it improves
Thank you for your response. No other livestock orher than n red hermit that has been with him for a long time. Aded some of my corals from my display only softies. Mushrooms and leathers. I checked on him this morning and all seems back to normal again. The tank is outside and with yesterdays temp the tank reached just above 30. Have no idea how to cool it down though. But my apple is still doing fine. I figure it just hurt itself maybe.
Franna, if your tank temp reached 30 degrees this could have been a cause for the fish gaping. The amount of dissolved oxygen ( solubility of oxygen) is reduced as the temperature increases, so you need to find a solution to bringing the temp down in any event. The corals in your tank are slightly forgiving of temp's above 28 deg C, but will not tolerate this on a continual basis.
If you cannot invest in a chiller or find some other mechanical method of covering the tank etc., my advise would be to ensure agitation of the water surface to speed up evaporation. The evaporation will help cool the tank volume slightly.