I am worried about my male Betta splendens I recently noticed a marble-sized swelling in his abdominal area. He is eating well, flaring and being his everyday self and so far does not seem to be affected by it. He lives on his own in a 20-gallon aquarium with nitrite, nitrate and ammonia all 0, temp is around 26 degrees Celsius. I use one of the slimline Castle Dawn Aquatics air-driven filters, as he is a half-moon Betta so I didn’t want a heavy flow in the aquarium. His diet consists of freeze-dried water flea and Hikari Betta pellets, occasionally he will take flake food. I know it's not dropsy from experience so what do you think is wrong with him?
The swelling you have described could be down to several problems. The swelling of the abdominal area in fish can be a sign of internal disease particularly tuberculosis and an internal fungus caused by Icthyophonus. Both these diseases can affect the liver and kidney functions which causes fluid to build up in the abdominal region. From the outside, it can look like simple bloat but as time progresses it becomes more apparent what the ailment is, as they are both wasting diseases.
In some cases, the abdominal swelling can be caused by Hexamita, which is common in Betta, like the early stages of tuberculosis and Icthyophonus it is often mistaken for bloat/constipation. Hexamita is a microscopic parasite that lives in the gall bladder and intestines of fish, which can cause swelling before leading to Anorexia, weakness and eventually death. Have a close look at your Bettas vent, if the vent appears to be slightly protruding and red, or the Bettas skin tone darkens in areas, it’s a clear sign the betta is suffering from Hexamitiosis.
Since your fish is still in good health there is a chance that he is suffering from constipation especially since his main diet consists of dry and freeze-dried foods. Most would recommend shelled peas for constipation. However, from personal experience keeping Bettas for nearly 2 decades cater to their micro-predator needs, and feed them some live Daphina or Mosquito larvae, you will find they are far more efficient at relieving constipation than shelled peas.
What I suggest is you try reliving possible constipation first with live foods, and see if the swelling goes down. If your Bettas health starts to deteriorate, goes off his food or becomes lethargic then there is something more serious afoot.
Sadly with ichthyophagous and tuberculosis, there is no real treatment other than making your fish comfortable. However, for internal parasites like Hexamita treatments containing Metronidazole will do the trick. Metronidazole will do a number on your beneficial bacteria, so ensure extra oxygen is being pumped into the aquarium. If it is Hexamitiosis the whole aquarium will need to be treated as the Hexamita parasite can linger in the aquarium. Alternatively, treat your Betta in a sick tank and deep clean the aquarium and decor with Castle Dawn Aquatics reset tabs.