Is this Velvet, otos and tetra

[Stunning fish]

Got Six otocinclus and 3 glofish tetras, want to mix the glofish with normal silvertips in hopes of watching the genes spread, 

10 Gallon quarantine tank, running for 2 weeks before purchaces

they are going into a 44 gal corner tank, currently gaining algae. Hosts 7 platys and 3 mollies

1 platy is in their own tank recovering from fin rot, unsure of the cause, but is recovering.

been feeding a mix cucumbers, nori, algae covered rock/hides.

frozen daphnia, and live moina for tetras but 1 oto will go after them.

seachem paragaurd is what i chose to treat with during quarantine. I have other medications on hand, sulfaplex metroplex, kanaplex,

June 8 I purchased the fish

1 oto died within 24 hours of getting them, didn't notice anything wrong with them during examination, round belly. No marks that i could see, this one also never calmed down, in the bag, the bucket i acclimated them in, or in the tank, didn't notice this behavior at the store.

June 18

2nd oto died by jumping out of tank, found them behind the tank between the glass and the foam mats i use to darken the tank, put them back in some water, had some very slow breathing, but after 2 hours they had left us. Had jumped through a gap left when i had left the lid ajar. No idea how long they were out of the water. Had what i thought was a nipped fin, but their gills was a dull grey color.

Inspected all the fish with flashlight and saw goldish marking on all of them, including their gills. including the deceased, 2/5 ottos had damaged tail fins. 1 tetra had a damaged tail fin

Assumed velvet, so increased temp (31) slowly and darkened tank, along with twice a day quick vac of the bottom of the tank, usually works out to <10% water change.

This is when they began breathing quickly, increased air and continued dosing paraguard.

today, June 29 noticed these growths(?)  on some fins, reviewing photos i see it started on this one oto, before the 25th, but was a very small bump. Now 3 of the 4 otos have damaged tail fins, and 2 with these round nodes either cream or red colored.

Is my diagnosis of Velvet incorrect? 
is the tail fin aggression or damage from a parasite?

paramaters,

Ph around 7.4

Kh around 150

gh around 300

Ammonia 0

nitrite 0

nitrate 20, source is usually 5-10

Edited June 30 by Stunning fish
Forgot parameters

[nabokovfan87]

It looks like fin rot, getting very close to body rot.  You'd want to add aeration, botanicals if possible, salt if you can given that there might be a risk to plants, and kanaplex to treat this.

It's a type of gram negative bacterial issue.  There's also what looks like some cysts or something.  You might have to do 2-3 treatments of the kanaplex and then follow up with something secondarily if you see any further issues on the fish.
 

On 6/29/2023 at 7:13 PM, Stunning fish said:

want to mix the glofish with normal silvertips in hopes of watching the genes spread, 

Just a heads up that you shouldn't try to crossbreed glofish with anything.  I do not think they will breed, but just mentioning it if you're planning a bigger breeding effort in this regard.

On 6/29/2023 at 7:13 PM, Stunning fish said:

2nd oto died by jumping out of tank, found them behind the tank between the glass and the foam mats i use to darken the tank, put them back in some water, had some very slow breathing, but after 2 hours they had left us. Had jumped through a gap left when i had left the lid ajar. No idea how long they were out of the water. Had what i thought was a nipped fin, but their gills was a dull grey color.

This points towards specifically some sort of water parameter issue.  It could be temperature, aeration, or something like ammonia/nitrite/nitrates or PH causing stress.
 

On 6/29/2023 at 7:13 PM, Stunning fish said:

paramaters,

Ph around 7.4

Kh around 150

gh around 300

Ammonia 0

nitrite 0

nitrate 20, source is usually 5-10

PH is getting right on the border or potentially too high.  I would verify it with a second test of some kind if you can.  If you tap is lower KH, then just do a few healthy water changes, 2-3 days of 50% water changes.
 

On 6/29/2023 at 7:13 PM, Stunning fish said:

Is my diagnosis of Velvet incorrect? 
is the tail fin aggression or damage from a parasite?

It is very difficult to tell and it's likely a compounding of a few issues.  If you don't fix the fin rot/body rot, then nothing has a chance of healing.  I do not see velvet.  Velvet looks like a yellow or gold dust of sorts across the surface of the fish.

Example: