Because I have two fish with the same symptoms, I am going to answer each question twice. If it is red, it applies to my male, Odysseus, whilst the green text is for my female Artemis.
What size is your tank? 5 gallons 16 gallons
What temperature is your tank? 82F 78F
Does your tank have a filter? Yes Yes, two
Does your tank have an air stone or other type of aeration? No, just surface movement from the filter Ditto
Is your tank heated? Yes Yes
What tank mates does your betta fish live with? RCS Cories, Endlers, Ember tetras and RCS
What type of food do you feed your betta fish? Cichlid pellets Betta micropellets
How often do you feed your betta fish? 3 pellets twice a day Twice a day
How often do you perform a water change? Once a week (or fortnight if I'm studying because I get so busy :( bad me) Once or twice a week, maybe less if I'm studying (again, bad me)
What percentage of the water do you change when you perform a water change? 50% 20% (more if I've been slack lately)
What type of additives do you add to the water when you perform a water change? Prime Prime, and occasionally a plant supplement
Have you tested your water? If so, what are the following parameters?
The following applies to both tanks
Ammonia: 0 (we have soft water so it gets converted to ammonium anyway)
Nitrate: 25ppm (slightly more in the 16 gallon)
Hardness: Really soft
Symptoms and Treatment
How has your betta fish's appearance changed? Both fish are lethargic. Artemis is dark, Odysseus is pale. Neither will move or eat. Artemis was pale and swollen, although that has reduced. She has clamped fins.
How has your betta fish's behavior changed? Extreme lethargy and refusing food.
When did you start noticing the symptoms? 5 days ago.
Have you started treating your fish? If so, how? I moved Artemis to a 3 gal quarantine tank with IAL and epsom salt, as she was bloated. Her colour has darkened back to normal (she was pale and stripy) and her bloating has reduced but no other improvement. I have not done anything for Odysseus except a water change.
Does your fish have any history of being ill? Nope.
How old is your fish (approximately)? Both fish are about two, Artemis may be a little older
Please help if you can. Pretty much every betta I've owned has died of this mystery ailment - they just slowly fade away, and one morning I will come down and find them dead. :( If you have any more questions, please do ask. :)