What is the temperature of the tank? Are you relying on the temperature of the dial on the heater or do you actually have a thermometer saying the actual temperature. Please do not rely on the scale on the heater as they are notoriously inaccurate and can be off by as much as 3 degrees in any direction even though they will tell you they are not on the box.
This is also getting into warmer weather in some areas of the country and the tanks can become over hot just being in a room that is warm. Bettas do not do well over 81 degrees or under 78 degrees Fahrenheit.
What are you feeding him? And how much at a time? Bettas tummies are the size of their eye and it is so easy to overfeed them as they have no discipline when it comes to when to STOP eating. They are not like most fish and they do not stop when they are full. It is not safe to give them enough to allow them to eat for 2 or 3 minutes and then remove the rest as most pellet foods or freeze dried foods expand in their tummies and will overfill them to the max. I soak the foods I give my guy for at least 5 minutes before giving it to him at all to take care of most of the expansion problem and also to allow it to regain its natural state. (freeze dried bloodworms in dechlorinated water with a drop of Vitachem) He is allowed to have maybe 10 bloodworms at a time this way and that is ALL. He is not starved and is fairly healthy. Most fish are obese and overfed anyway in tanks as they do not get the amount of exercise they would get in the wild so they need to be controlled in their eating habits. It is also a good idea to either fast the betta for one day a week with NO food whatsoever to keep the digestion process from causing constipation or preferably give them defrosted peeled frozen pea bits that are half the size of their eye (2 or 3) a couple of times a week. If you give them to them when they are hungry the first time you will find they love them and will never have to coax them to eat them again. Diablo would rather have peas than anything. He adores them!
I am betting the water is either too warm, too cool, or he is constipated or overfed. That is usually the cause for the hiding in the corner. How is his color? If his color is off, then it is almost certainly a digestive issue of some type and peas and fasting for at least 3 days are definitely in order. I have seen bettas that are near death from constipation come back with this treatment. But this means that you have to watch how much you feed them from now on as a betta who has become constipated will be more prone to it in the future. Small protein meals and no more than two meals a day. The frozen thawed pea treatments at least two times a week and probably at least one meal a week missed to allow their digestive system to rest.
Pea treatment instructions:
Take a single frozen pea from the bag and place on a saucer with no water.
Place in the microwave for about 20 to 30 seconds on HIGH.
Remove from the microwave and allow to cool and remove the peeling from the pea.
Take one half of the pea and with a sharp knife cut small cubes of the pea the size of half the fish's eye and give the fish 2 to 3 of these from a clean fingertip floated slowly into the water. this may take a little practice for them to realize it is food but do not distress yourself if they do not realize it the first time. If they are hungry they will figure it out.
If they nip your finger it does not hurt. I have had Diablo nip me and it does not hurt much. It is about like being pinched and you will soon learn to just let the pea piece float off the tip of the finger and let the fish chase it. Just do not do it too close to the filter outlet where it will disappear too fast.
A chunky little cube works better than a slice they can get ahold of it better but not too big or they cannot get it in their mouths.
Hope this helps. Please do let us know how he does and I would get a stick on the outside thermometer if you do not have any other one. They are surprisingly accurate. and cheap.
get one that is big enough for you to see well.
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