Sick betta, pls help!

[Kerryjoy]

Hello

My betta is not himself, but he is eating.

He is lethargic and just drifts around. Spends time hiding at bottom of the and breathing. 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!

I’ve done 50% water change this week.
Flow is very low if not non existent.
Other 2 fish are happy and healthy.
I’ve added and air pump to help the water in the last week

Pictures below

 

[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?