Betta emergency

[Kerryjoy]

Hello

My betta is not himself, but he is eating.

He is lethargic and just drifts around. Been like this for 2 weeks now but he is still ok and eating when he sees me come, he is trying.
My water levels are fine, he is in a 38l tank with 2 corydoras, I have had him for half a year now and he has been doing very well. The other fish are happy and fine. The tank has a filter.
I change 50% of the water once a week and clean the sand too for any uneaten food.

Nitrates 25
Ph 7.6 (how it has always been for a year)
Anything I am missing?
Any ideas please, I would like to save him as he looks so sad atm poor fella!

Pictures below

 

[Colin_T]

What are you feeding him?
Dry foods can cause them to float about and have balance problems due to the air in the food. If you don't feed frozen or live foods perhaps try some live brineshrimp or frozen prawn. Just defrost a prawn, remove the head and shell, then use a pr of scissors to cut the prawn into bite size bits. Offer 1 or 2 bits at a time and stop feeding when he is full. The other fish will also eat prawn and brineshrimp.
You can also feed him small insects like flies, mosquitoes, aphids. Just make sure they are free of chemicals like pesticide.

In one of the photos he appears to be floating on his side. Does he normally do this?
Floating on his side is usually air in his intestine and normally associated with dry food.

 

[NickAu]

Can you please answer the following questions.

How big is the tank?
Has it got a heater? What is it set to?
Has it got a filter?
How strong is the current?
Is it cycled?
How often do you change water?
How much water do you change?
Do you vacuum the substrate?
What additives or chemicals do you use? Eg De Chlorinator?
Do you use tap or bottled water? If bottled water please give us the brand.
Do you have a water test kit?
Can you tell us the readings for ammonia, nitrite and nitrate?
When you clean the tank can you tell us exactly what you do.
What do you feed him? how much? how often?
Any tank mates? What sort how many?
How long have you had the Betta?
Has he got a history of illness?
Any plants or decorations? Please give details?
Can you provide a close up photo of the fish?
Can you provide a photo of the tank?