Help Siamese Fighter Unknown Health Issue?

Is the tank cycle? What are the actual readings for ammonia, nitrite and nitrate?
How long has the tank been set up, and how long has the betta been there?
What size tank?
Have you seen the danios pestering him? Danios can be aggressive if there aren't enough of them (they need at least 6) or in a tank that's too small.

How often do you do water changes, and how big are they?

If the betta has been in the tank only 4 weeks, it is not likely to be old age.

To be honest, I'm not sure what to suggest. The water conditions look fine - well, nitrate is a bit high, but not high enough to cause problems yet.
Nitrate should be kept below 20 ppm, which is why I asked about water changes. Though I am aware that many places have nitrate higher than that in tap water which makes to difficult to get tank water under 20.
The water is possibly too cold for the betta - they like it around 26 or 27 degrees, but this is too warm for danios and cories, both of which need under 25 deg.




But for the sake of the fish I do need to point out a couple of things, which you may not like.
Danios and cories are shoaling fish which need to be in a group of at least 6, with more being better. But your tank is too small for this. Danios are incredibly fast swimming fish and need a long tank because of this - 90 cm minimum. Unless the cories are one of the dwarf species (pygmy, habrosus or hastatus) they also need a bigger tank - and sand or small smooth gravel on the bottom. Sharp angular gravel can wear down their barbels, and trapped decomposing food can then lead to infections in the damaged barbels.

The good news is that the tank is fine for a betta 

With a 39 litre tank, you should be looking to do 15 to 20 litre water changes a week - around 50% of the tank volume. Increasing the amount you change could perk up the betta. Fish, like humans, excrete and secrete a whole host of things - I used to work in a hospital lab and you'd be surprised how many things we tested in urine. In a fish tank, these build up in the water between water changes, including 'communication' chemicals - pheromones to communicate with their own species and allomones to communicate with other species, and these can be aggression signals. So in a tank, there is chemical aggression as well as physical. Large water changes reduce the amount of everything made by the fish better than small water changes.

So many shop workers haven't a clue about fish care, so it's not surprising they didn't tell you about the danios and cories when you got them 



Do you have any live plants in the tank? Aquatic plants take up ammonia as their source of nitrogen and while this won't help with nitrate in the tap water it will help prevent the tank nitrate level increasing due to filter bacteria making it from the ammonia made by the fish. Floating plants are particularly good for this as they are close to the lights and they can get carbon dioxide from the air.