Black skirt tetras seem scared and skittish, one has a black Mark on the base of his tail fin

[LinearParadox]

The only thing that has changed is I added a Bolivian ram female. She’s not being aggressive, and mostly just exploring the tank and chilling. Before, they would swim freely at the top and not demonstrate any schooling behavior.

 

[mcordelia]

Hmm! How big is the tank? How old are the fish? Apart from the black mark, has their coloration changed otherwise?

What are your water parameters?

Now annoying question: are you absolutely sure nothing has changed with the tank other than the ram? No decor changes, filter cleanings, new water routines, people in the house moving around the tank, etc etc?

When did the ram get added? Did you quarantine it first? How many tetras in total are there?

 

[LinearParadox]

Hmm! How big is the tank? How old are the fish? Apart from the black mark, has their coloration changed otherwise?

What are your water parameters?

Now annoying question: are you absolutely sure nothing has changed with the tank other than the ram? No decor changes, filter cleanings, new water routines, people in the house moving around the tank, etc etc?

When did the ram get added? Did you quarantine it first? How many tetras in total are there?

Black skirt tetras are a few months old. They’ve been doing well for a while. It’s a 29 gallon, and they do lighten up when they get scared (usually when I do a water change it freaks them out).
Water parameters are 0 nitrites 0 ammonia 5 nitrates. I did a water change shortly before adding the ram (I know they can be sensitive, wanted to water to be extra clean).
Ram was added day before yesterday. I didn’t quarantine it I unfortunately do not have the space. There are 9 tetras. They seem to be doing better today, still schooling but not as freaked out and skittish. I think the water change and the ram in one dah might have freaked them out more than usual. I still don’t understand what that thing in the tetras take is though.

 

[CassCats]

The black mark on the tail could be fungal. Id treat with some aquarium salt in this case to see if it helps