Epsom Salt (no fragrance and no dye) is gentle and safe to use with plants.. but for best results/treatment, daily 100% water changes is recommended. Also a bare-bottom to keep an eye on waste. Do you have a small tupperware container you can use to secure at the top of the tank? Or a small QT tank to use during treatment?
It's important to keep him separated from his home tank that is set up such as that and cycled, not only to keep an eye on his waste- but to make the 100% water changes easier and if he gets worse and can't swim very well you can lower the water level down to a few inches.. and to save your cycle.
Almost anything is fine to use- as long as it's clean and there is no soap residue. Even the cup he came in would work. You can even use a gallon water jug cut in half to hold him in- anything with an open top I suggest placing seran wrap over the top held on by tape or a rubber band so it doesn't collapse into the water.. and a few holes punched in it for air.. they are quite the jumpers.
If you can't get it to float, then keep it in the warmest part of your home with a towel wrapped around the sides to keep the warmth in.
If the container is less then a gallon, you can use a clean water/milk jug (thoroughly rinsed out-no soap) to mix in the Epsom Salt, conditioner and water to make the daily water changes easier. Always make sure the salt is dissolved before adding your guy into it.
Epsom is safe for them.. it's a form of laxative that will help reduce swelling/bloating. You can get it at most pharmacies or stores with a pharmacy section.
If he goes to the bathroom, can swim normal and his waste looks a normal brown then you can release him back into his home 12-24 hrs after he looks/acts/swims normal.
If after 5-8 days and no improvement, let us know.. if his waste is white and stringy, let us know asap. If he starts to show signs of more illnesses, such as loss of color, gasping for breath, scales sticking up.. let us know asap. Hopefully he is just bloated.. but we won't know for sure unless he shows more signs. For now, the only thing that is best for him, less stressful and safe on his organs is the Epsom.