Large black spots on a new blue tang

[eseric]

Hello everyone,
I'm new to the saltwater aquarium hobby and have very little experience dealing with potential infections. I have a fairly new 160 gallon (+ 40 g sump) tank which I cycled via ghost feeding ( + live rocks I added to the sump). One week and a few days ago I went to an LFS and purchased 2 clowns, a foxface rabbitfish, a small royal gramma, and a small 1.5 inch blue tang, and a few snails and hermits. Unfortunately I did not QT any of the fish and also do not have a QT tank either (which I plan to get fairly soon).

The blue tang seemed healthy when I purchased it (active, colorful, no spots) but the last 3 days I've been noticing a large black spot (surrounded by some de-coloration and some smaller black spots). It only appears to be on one side. It's also been scratching from time to time against the rocks. Besides the apparent physical bruises, the tang is very active, seems to have a good weight, and it eats whatever I feed it (frozen mysis shrimp, frozen algae marine sea food, grazing on nori sheets, etc). I recently started soaking some of the food on selcon since I read that helps with immunity. The rest of the fish seem healthy (except the royal gramma which I haven't seen since I added it).

My basic water parameters seem to be within range.
Salinity - 1.025
Temp - 78.5
PH - 8
Ammonia - Hard to read API test color chart but it seems between 0-.25ppm
Nitrite - 0
Nitrate - 5 ppm

I'm posting a couple videos here as well as images. I would appreciate if someone could chime in on what the best course of action is (not sure if it could be black ich, flukes, etc).

Thank you in advance.

Videos:


Pictures showing black spots:





Other side:

 

[Dread Pirate Dave]

I'm not sure off hand but there are others here with much more knowledge that can help.

 

[Jay Hemdal]

You may have two different issues going on with the tang. The scratching is pretty often a result of flukes (external worm parasites). These can be treated using Praziquantel, dosed 2x, 8 days apart (sometimes a third treatment is needed). You need to increase aeration during the treatment, remove any chemical filtration, and run any skimmers, just don't collect the skimmate.

Now - the lesion on its right side - that isn't from flukes, more likely some injury that has gotten infected. Foxface have long dorsal spines that they can use in defense, I wonder if the tang got spiked?

 

[vetteguy53081]

That circle generally ive seen from as mentioned foxface spine, urchin or even a bite from another tankmate. I would also encourage you to take a water sample to an LFS that does Not use api kits and see what readings they come up with and to compare with yours

 

[eseric]

Thank you all. Yeah that makes sense, especially given that the large injury was pretty much overnight. It might have gotten into a scuffle with the foxface. I'll treat it with PraziPro as recommended here and will report back. I'll also take a sample of water to an lfs around here for testing. Appreciate it!