There is a possibility that this could be something more sinister than simple constipation-bloating. Definitely do not feed him--epsom salt (magnesium sulfate) may help reduce the swelling and encourage him to defecate. There are two ways to treat your fish using epsom salt:
Epsom Salt Bath: In a separate container, prepare water of the same temperature or use water from the betta’s tank and dissolve 1 tablespoon of Epsom salt per gallon. Slowly adjust the betta to the water over about 5 minutes--acclimation time is much shorter because the fish shouldn’t be exposed to this amount of Epsom salt for too long--then allow the betta to sit in the bath for 15 minutes. Carefully monitor the betta, and remove him or her if they show signs of distress. Consider changing the water in the betta’s tank during the bath so that the fish will have clean water to recover in. This treatment can be repeated once in the same day, but if repeated, the duration of the bath should be shortened to 5-10 minutes.
Epsom Salt Leave-in Treatment: In a separate container, prepare water of the same temperature, and mix in one tablespoon per five gallons of dissolved Epsom salt. Acclimate the fish to the salted water for at least a half hour. Alternatively, if you used water from the betta’s tank, you can transfer the betta into the water directly and dissolve the salt in another container--slowly adding the dissolved salts to the water over the course of at least a half hour. The duration of this treatment is three to five days--during this time, do water changes as necessary, replacing the dissolved salts as necessary.
This is very extreme for food related bloating, so I feel that there is a strong possibility that this is Malawi Bloat/Mystery Bloat. The epsom salt should still be helpful if this is the case, but I haven't heard of a single case where a betta has recovered from bloating related to this pathogen. I hope that your fish has just been overfed, but due to the possibility that he has an infectious disease, be very careful to avoid cross-contamination so that your other fish doesn't catch it. Don't use the same nets or cups or any other tools like that in both tanks without sterilizing them first.
Last edited by Adastra; 09-28-2010 at 12:31 AM.