No, it doesn't need to be filtered. You'll just need to do more frequent water changes, since it's a small unfiltered tank.
OK, so it's a 1.5 gal tank, right? There are many ways to do a water change and acclimate your fish back into the tank of treated water so his system isn't shocked. If you have a procedure that works, great! If not, here is one variation:
- Note the temperature of his current water.
- Using a plastic cup, scoop him, along with some of his water, into the cup.
- Fill the 1.5 gal hospital tank with water at the SAME temp. Be sure to add the correct amount of water conditioner.
- Add 1.5 teaspoons of Epsom salt. (This will give you an overall dosage of 1 teaspoon per gallon.)
- 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.
Leave him in the Epsom salt treated hospital tank. If he isn't better in two days, do a 50% water change:
- Mix 1 teaspoon Epsom salt into 1 gal of water at the same temp as his tank. Add conditioner. Mix until the salt is dissolved.
- Remove about 50% of the water from the hospital tank.
- Slowly add the new Epsom salt treated water. (You can add 25%, wait about 10 min, then add the other 25%.)
Hopefully, the Epsom salt will relieve the bloating, and help his buoyancy issues. Also, monitor for poop.