I think you forgot to upload the picture you referenced.
I've gone back thru the thread to see if I have any other ideas.
Brooklynella was the initial diagnosis and formalin or acriflavine (ruby rally pro) was the recommended medication. You followed the acriflavine recommendation.
I see a couple of possibilities. One, the Brooklynella was too advanced and the acriflavine didn't have time to work. Second, Brook was the wrong diagnosis. The other likely culprits are ich and velvet. Velvet usually results in rapid decline and death. One constant with velvet is very rapid breathing. I could never get a clear enough video to try to count breaths to estimate the breathing rate. Since the illness persisted for several days, I'm going to assume velvet was not the issue. The other possibility, ich, can linger for days without causing death. Decline in health is noted as spots come and go early in the infection with more and more spots developing over time. The fish will also become lethargic and may begin flashing as the parasite gains more and more control and spreads to the gills.
Treatment for ich is copper power or coppersafe. While the possibility of Brook continues to exist, if you have not noticed any improvement but instead continued decline, I would switch gears. You will need to get to the therapeutic copper dose of 2.50 ppm quickly. I would add 50% now and 50% more in 3 to 6 hours.