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Sick danio deformed, sunken belly, swimming in circles


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I hope a video link is allowed...I have a question about what's making this fish sick and a video is just more helpful than a photo because it shows his behavior as well.

Backstory: I got 12 Danio nigrofasciatus from Wet Spot Tropical Fish last year (maybe June?)  Slowly they've died off and now I only have 4.  I only ever saw 2 of them get sick, plus this one (so 3 in total), and I only ever found one body.  The others just kinda diseappeared.  I normally do 20-30% water changes on this heavily planted tank about once every 2 weeks.  There was a period where I was very busy with work and I missed water change for probably 2-3 months - not great, but that's life sometimes.  I can say that I never measured nitrates over 20ppm in this tank (even during that period) and ammonia, nitrite have always measured 0 ppm.  I assume the pond snails made quick work of the fish that have died.  During that time some of the danios disappeared.

Parameters now: ammonia, nitrite, nitrate 0 ppm; hardness 25 ppm, buffer 40 ppm, pH 6.4.

About the tank: it's a very heavily planted, 17 gallon tank with ADA aquasoil substrate (which is a buffering substrate), and a little bit of pressurized CO2.  I fertilize with either Aquarium Coop or NilocG 2-mL about 1-2x/week.

Fish behavior: hopefully those interested are able to view this 30-second video at the link below.  This is pretty much what she does all day since yesterday.  More circular behavior today for sure....yesterday she was occupying more regions of the tank and also eating.  You can see the weird spine shape - that's not how she was before.  You can see also the weird "sunken" belly (is that what sunken belly is?)  Compare to the healthy ones that swim by.  These guys get Xtreme Nano and BBS 1-2x per day.  Also of note, she gets what appear to be crystal clear little "air bubbles" on the left side of her, near that sunken part of her belly.  Almost like air bubbles escape from a hole in her side.  3-4 will appear, and then after a while they'll be gone.  That's happened twice that I've noticed since I started looking closely yesterday.  She also has discoloration...not sure if it's obvious from the video or not, but she's very "dull" compared to the others.  This exact thing is what happened to the other two that I saw that died.

I know this fish is going to die.  To be honest, I typically just don't mess with fish medication because it has always been way too confusing to me.  I am one of those people who will pay more for fish in exchange for my pet store doing the quarantining and treatment.  I have a tank that was intended as a quarantine tank, but it currently has no heater and also houses extra plants.  I just want to know if this is Mycobacterium or not, and if not, I'd love any suggestion as to what it could be.

Link to sick fish video:

Thank you, everyone!

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