I have kept Bettas off and on for about six years with good lifespans on any fish that survived the first month home from the pet store - with one exception. I brought Indigo home about two months ago and he seemed fine. The other Bettas we bought at the same time died pretty quickly, but he kept eating and swimming and did okay. After a few weeks I noticed a patch of fungus, but I treated it with a product by Jungle that I have used before and it went away. He seemed okay again. About three weeks ago, that all changed. He stopped eating, stopped swimming so much, started getting VERY sensitive to vibrations under his bowl, and then started floating on his side a lot. I've tried water changes and fasting which I've read are good for swim bladder issues, I can't find any external parasites which I've read could cause the erratic darting around, and I can't find anything online that mentions a sensitivity to sound or vibration. We're talking, like, if you close the door of the cabinet under the counter he sits on (gently, and it has felt pads on it) he goes nuts.
He has a 3/4 gallon bowl
Weekly water changes (as with all my Bettas over the years)
Tap water treated with water conditioner (same brand as always)
No heater, but I'm So. CA and he's in the kitchen near cooking heat (other Betta is also and is fine)
He's alone in his bowl
He has glass marbles for substrate
No filtration, but I use a baster to remove debris
I feed pellets (same as for all my Bettas, and he ate them fine before)
No discolored or fuzzy spots or fin deterioration
I even cleaned out a different bowl (I have a few that are all the same) and a different set of glass marbles and used a net to move him to make sure he got a COMPLETE water change two days ago, and although I dropped a pellet in his bowl every day or two for the last two weeks, I found them all in the bottom of his old bowl so I'm pretty sure he's not eating, and hasn't been. Does anyone have any idea what I can do for this poor guy? He's beautiful, colored like a well-loved pair of jeans, and I don't want him to die.