1) Water Changes
Ammonia, waste and toxins build up in small tanks pretty quickly. Since he's only in a 0.5 gallon tank, you will need to change the water frequently. A half-gal tank will need almost daily water changes.
Your 1.5 gal tank will need at least one 100% and one 50% water change per week. (Many people recommend one 100% and two 50% changes per week for a tank that size.)
2) Food
What brand of food are you feeding him? Some bettas can become constipated and bloated on foods that contain a lot of grains/filler (corn, wheat, gluten, soybean, etc.)
There are two frequently recommended foods on this forum: New Life Spectrum betta pellets and Omega One Betta Buffet pellets. Both contain quality proteins, and fewer grains/fillers. Personally, I prefer pellets, since I can count them out, and I find them less messy than flakes. But some people like the flakes, as well. The important thing is that you feed him a high quality food.
3) Temperature
It's good to have a thermometer such as
this one. Bettas are tropical fish, and do best at temperatures in the 76-84F range. If the water temperature falls much lower than this, it slows down their metabolism (including their digestion). Low water temperatures can contribute to issues like bloating and constipation.
4) Suggested treatment
Since he looks pretty bloated right now, I would personally try Epsom salt. It has a laxative and fluid reducing effect. I would start with a dosage of 1 teaspon Epsom salt per gallon.
Do not use Aquarium salt. It contains sodium, which can contribute to bloating and puts stress on the kidneys.
There are many ways to do a water change, add Epsom salt, and acclimate (reintroduce) your fish back into the tank. Here is one variation:
- Note the temperature of the water. (Use a thermometer for this.)
- 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 the correct amount of Epsom salt.
Your 0.5 gal tank will need 0.5 teaspoons Epsom salt.
- 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.
- Repeat the above two steps, until an hour (or more) has passed.
- If the cup starts to fill too much, remove a SMALL amount of water from the cup. Discard it.
- Gently release him into the tank.