Like, can't reinforce this enough right now go get an air stone and air pump.
Do NOT buy a bunch of crap liquid potions off the shelf at Petco and start putting it in your tank to try and treat this fish. You need immediate water flow and to disturb the top of the water with flow so that carbon dioxide can release out of the water and oxygen can get in it. An air stone will help in this emergency situation.
Tomorrow do a +/- 15-20% water change. This means you need to start getting ro/di water ready right now if you don't have any on hand, and mix it in a bucket with your salt mix to get it up to correct salinity 1.026.
If you don't know what ro/di water is you need to spend 15 minutes googling it and why it's important for use in a saltwater aquarium.
I would highly recommend you start reading some of the sticky threads in the beginners section of this forum.
If you don't get an air stone in this tank now, your fish will be dead in 48-72 hours.