Betta Fin Rot?

[Matt_]

My female betta has developed an infection over the last two days after I brought in some rummy nose tetras that did not survive.  She has lost the top of her tail fin and has a white mucus developing on her side.  She is very lethargic and not eating.  This has all developed since Friday night when she was not displaying any symptoms.  The infection looks like fin rot, but I would like to get a second opinion since I am not a disease expert.

I also lost an adult female guppy out of the tank this morning, but the rest of the fish in the tank appear to be healthy.  I changed the water, added Prime, and started treatment of the entire tank with E.M. Erythromycin today.  I have also separated the betta in a breeder box with a USB nano air pump and some Watersprite that is located away from the light.  Recommendations are appreciated (I have the rest of the med trio and salt on hand).

Before Illness on Left exercising with the male in a Lee's Specimen Container on Right (her fins were originally torn by the male and never returned their original shape):

 

Current:

 

Water parameters (API Liquid Test):

- Temp: 76F

- Ammonia: 0.25-0.5 (I added more Prime after this test)

- Nitrite: 0

- Nitrate: 40

- PH: 6.6

- GH: 7 deg.

- KH: 2 deg.

Tank:

- 20 Gal long Bare Bottom with sponge filter setup for over a year - Last major change was transferring the entire tank from a 10 gal. to a 20 gal. a month ago

- Tank mates:  Single female betta (sick), Guppies, 5 Beckford Pencilfish, and 4 Bristlenose pleco growouts

[Colu]

It looks like bacterial infection I would keep treating with maracyn if you see no improvement I would treat with kanaplex as it could be a gram negative bacterial

Edited March 28, 2021 by Colu

[Matt_]

Thanks! My big concern is that she will not make it though tomorrow if I do not make the right call right now.

[Colu]

It difficult to no for sure whether it's a gram negative or positive bacterial infection all you can do is treat  the symptoms they are showing 

[Matt_]

I has been 24 hrs since I started treatment with the Erythromycin and the betta appears to be getting worse.  The white mucus spread to her moth and I not found a way to get hold of KanaPlex today.  Any other suggestions would be appreciated.  I am thinking of starting a salt bath for her.

[quikv6]

Yes...salt, especially if you have no other options. I would continue with the Erythromycin, as sometimes that can take some time to see a rebound.

[Colu]

You could Qarantine and keep treating with maracyn and add aquarium salt 1 table spoon for 2 gallons

[Matt_]

Thanks.  I added the salt and will keep using the Erythromycin.  Unfortunately, she is extremely weak now and is probably not going to last the night. 

[Matt_]

17 hours ago, Matt_ said:

Thanks.  I added the salt and will keep using the Erythromycin.  Unfortunately, she is extremely weak now and is probably not going to last the night. 

Update, she passed away last night.  I am so sad having lost my pet fish that would eat out of my hand due to human error (failure to QT new arrivals) 😭.

[Colu]

Sorry to hear he didn't make it