Do you have Epsom salt? It should be 100% magnesium sulfate. (No dyes, perfumes or additives.) It's available at pharmacies and supermarkets. (But not at pet stores!)
Since he's having trouble swimming, you might want to move him into a smaller, shallow tank or bowl. Or you can lower the water level in his 10 gal tank so that he can reach the surface more easily.
To treat with Epsom salt:
1) Scoop some tank water into a cup.
2) Add 1 teaspoon Epsom salt per gallon to the cup. Stir until the salt is completely dissolved. For example:
1 gal tank = add 1 teaspoon Epsom salt
5 gal tank = add 5 teaspoons Epsom salt
10 gal tank = add 10 teaspoons Epsom salt)
3) Slowly pour the Epsom salt solution into the tank, over the course of about an hour.
Even better, I would move him into a shallow, 1 gallon container/bowl/tank, and do the following. This will provide a 100% water change for him, and allow him to reach the surface more easily:
Note the temperature of the water.
Using a plastic cup, scoop him, along with some of his water, into the cup.
Clean out the tank. Refill with water at the SAME temp. Be sure to add the correct amount of water conditioner.
Add 1 teaspoon of Epsom salt per gallon of water. For example:
1 gal tank = add 1 teaspoon Epsom salt.
2.5 gal tank = add 2.5 teaspoons Epsom salt. Etc....
Stir until the Epsom salt is completely dissolved.
Float his cup in the tank for about 15 min.
Add a SMALL amount of NEW water to the cup. (Several tablespoons, or about 1.5 ounces.)
Let his cup float for about 10 min.
If the cup starts to fill too much, remove a SMALL amount of water from the cup. Discard it.
Repeat steps 5-7, until an hour (or more) has passed.
Gently release him into the tank.
Also, cover the tank with plastic vegetable wrap. This will increase the humidity, and allow him to breathe more easily.
Edited to add: Epsom salt at this dosage will not harm plants.
Well, he was more active yesterday, now he has been floating sort of on his side at the bottle not doing much. I emptied half of the water out so he could reach the top better. Thinking I should empty more of it out today? I feel so bad for him. Should I do another epsom salt treatment?