A 3L (0.75 gal) tank needs more frequent water changes. I recommend doing at least two 100% water changes per week.
I would do a 100% water change immediately, using 0.5 teaspoons Aquarium salt. Acclimate slowly. (See below.) If he shows any signs of bloating, constipation, or buoyancy issues, do a 100% water change using plain (conditioned) water.
Until he is better, I recommend doing 100% water changes every other day. This will keep bacteria and toxin levels low.
If this doesn't take care of the problem within a day or two, let us know. (He may require a higher dosage of Aquarium salt, or an antibiotic.)
There are many ways to do a water change and acclimate (reintroduce) your fish back into the tank. Here is one variation:
- 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.
- Since your tank is 3L (0.75gal), add 0.5 teaspoons Epsom salt. Dissolve the salt completely!
- 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 about an hour has passed.
- Gently release him into the tank.