Is he floating on the top trying to get oxygen? Does it seem like something in the water is causing him to be uncomfortable? I had the same problem with my fish a day ago. I don't have beta but after doing a lot of reserach, I've learnt that one of the reasons they float on the top is to get more oxygen because something in the water makes them unable to breathe there. It could be that especially since you said he used to swim all the time. If that is the case, you need to increase the flow of oxygen, add bubbles. I even removed the tank lid completely for my fish. I don't know what temp Betas like but keep it cool because hot water is suffocation. I had to do 4 water changes in one night. My fish were floating on the top....all 4 of them....it was scary..but after the water changes...removing the lid for more oxygen slowly they started swimming in the water again as normal. I hope this helps!

Bettas can breath from air outside of water so not oxygen deprivation. MHO I think when you do a water change, and clean out the tank, you are essentially starting the cycle again ie good bugs. So too drastic a change all at once.

So just with this small amount of information could be ammonia spike (main reason for fish at top often) They are being starved of oxygen but due to gills having been burned. But bettas are air breathers; "they use a unique organ called the labyrinth to process the oxygen in the air and they will die if they are unable to reach the waters surface."

I think in this case though it is stress from too drastic a change and again, not knowing how much water was changes, perhaps a water chemistry problem which would stress him and open him up to bacteria problems if left unnatended.

So more information needed please. Do you do a partial change? Do you clean the filter at the same time? What type of filter? Any plants (they love live plants and Annubias which can just be floated is great, or those bunch plants which are cheap and can float). What else is in the tank? Sand gravel (not really needed for such a small size)

Sometimes they need a cave or coffee cup to have a hiding and or sleeping place. But because this occurred after a water change I think there is to be found the answer. Was the temperature the same, ph, hardness so on. Sometimes with Bettas I had a pinch of salt and stress coat as I notice the water changes have to be precise (let us say) I have best luck with dribbling it in from that little air line tubing with a knot in it, ie very slowly over time. If I don't do it that slowly mine gets stressed as well. I am ordering the Indian Almond leaves which hardens their scales so as to help with mine getting flexibar if the change is not slow enough. I think they are stressed from the way they are bred, shipped and treated (horribly) in the stores (I saw my local store taking them out of those little baggies with about one tablespoon of water, and twirling them all around, I almost threw up!) Like objects.

So more info please, and meanwhile you can add some salt (they can tolerate a lot for treatment purposes if added slowly over time) but just a little for main water to help with any osmotic problems from water change. Some plants for security, and see if any ammonia spikes. Best Sherry

Also, since you've had in for just a while, and most likely he was in those cold little bowls (hopefully not for too long) any resistance may have been reduced due to this water change. Do you notice any thing on him? Scales with discoloration? (beg of flexibar which is common in betas) ragged fins edges? Anything or just floating.

Also, how much and what are you feeding. I am just asking to delete anything not pertinant, but think again most likely due to water chemistry.

They shock easily. And I found out lately my local water can change drastically in ph values. So now I check it (ugh) each time I do water changes with all tanks.

If it is depression, get him a buddy. You can put beta in tanks with other fast swiming, non-fin nipping breds such as just a cheap common golf fish, or small feeder goldfish. No one likes to be alone. Maybe this will help.