Anyone around to help with poorly Betta fish?

[Popitdontstopit]

Our Betta is sitting at the bottom of his tank. He swims to another part of the lower tank sometimes, but none of the darting about he did until recently. I saw him make it to the top last night which was good.
What I've tried: he is on day 3 of a 3 day fast in case of constipation.
I've put some "swim bladder" liquid into his water. This says to be repeated after four days.
The water levels are all fine apart from the KH which was 0/next to 0. I live in a soft water area which will influence this - not sure if this is a coincidence or what's upsetting the fish.
Any other ideas gratefully accepted

Have you done a partial water change?

What are his tank stats - the full water parameters plus tank size/filtration/how long it's been set up etc etc?

How old is he? They normally live about 3 years. Also, mine is mega lazy and often lies on the bottom but is absolutely fine.

I have a koi pond that has some very large goldfish in it. Last weekend one of them started swimming more or less on its side and upside down and very clearly had swim bladder disease. I Googled it and everybody recommended Epsom salt baths to relax the muscles and nerves so I thought what the heck I'll try it and after two days of a couple of hours per day putting the fish in an external bucket with concentrated Epsom salts and then a second lower dose Epsom salts I actually managed to get rid of the gas and the fish is totally back to normal. Google the dosage because it may be different for your smaller fish. But I used 1 tablespoon per gallon of water on the concentrated one and quarter tablespoon 4 gallon of water on the rinse bucket. I had Epsom salts around the house because I use them for fertilizing in the garden. Just make sure that you're not using something that has perfumes or additives