Most likely bacterial as that can cause problems with swim bladder functionality. If all other fish are OK treat him in a quarantine tank: Melafix/Pimafix combo, or eSha 2000 should fix it.
Are the Danios in your main tank? Just asking as the temp in there is likely to be a tad high for them. Also pH is a bit low and water rather on the soft side for Danios which may have lowered his immunity.
You're right actually, I went by an info card at our LFS temperature wise. The tank is 1.5C over their recommended requirements looking at the fishkeeping caresheets.
what was going to be my quarantine tank is now home to my betta permanently. I do however have a food safe coolbox which I used to transport the clown loach in when moving home (due to their spines in plastic bags).
With a constant temperature in our home approx 22-23C, can I use that to quarantine him in as it wouldnt need a heater. Should i also pop an airstone in there with a controlled stream of bubbles?
In terms of meds, I always keep white spot treatment in stock, however I dont have melafix or the other you mentioned. Our LFS is so limited on this island. Can you recommend one of the king british meds as they stock a wide range of them.
Yes should be fine for small fish like Danios as hopefully he'll only be in there a few days anyway. Get some mature media from your existing filter and pop it into the filter of the 25 litre tank so you have an instant cycle.