For never having dealt with many fish illnesses, I certainly seem to be getting hit with them now (posted about new rummynose recently). In a completely different tank and that I use different equipment in, I now have another sick fish w/ different symptoms.

My peacock gudgeon who I've had for about 4 months in my 5 gallon has developed large(ish) black spots mostly around his head, plus one along his side under his dorsal fin. I've noticed all of these develop in the last week or so. Any idea what this could be? Pics below. He's mostly acting fine and eating like a glutton, but I've seen him flash just a few times in the past couple days (never saw him do so before). I'm quite fond of this guy, so I'm hoping we can fix this

The only recent change that has been made to his tank is that last weekend he gained a male celestial pearl danio as a tankmate (not a fish I'd usually keep without a school, but my friend needed to get rid of it). So this disease has suspicious timing. I did not quarantine the fish before adding him, as I figured he'd be disease-free since my friend had the CPD for a few months without issue - definitely regretting this now.

Edit - the only other somewhat recent change was the additional of a nerite snail a couple weeks ago.

I've been treating the tank with Paraguard for the last 4 days with no improvement (in fact they've possibly gotten worse), so I'm wondering if I should switch meds. The problem is I have no idea what it is, so I'm not sure what to treat with.

Meds and related items currently on hand (though I'm happy to order something or run out to the store ASAP if none of these seem like the right one for this):
Furan-2
Prazipro
Kanaplex
Metroplex
API super ick cure
Paraguard (not helping currently)
aquarium salt

Tested tank stats on Wednesday, and ammonia & nitrite were both 0 ppm, nitrate 5 ppm. Temp is usually 74F. I'll test again after this post and update if there are any changes.

Any input would be greatly appreciated!

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Tank stats still look good, but I'm going to do a water change just to be safe.
temp - 74F
pH 6.6
ammonia - 0 ppm
nitrite - 0 ppm
nitrate - 5-10 ppm

Only thing I've come across that it sort of looks like is ammonia burn, but there's never been ammonia present when I've tested the water. Regardless, if it is some form of ammonia or other chemical burn, hopefully it'll heal on its own with water changes. If anyone thinks it could be something else, please let me know.

Hey, I came across your post while researching this EXACT THING in my own male peacock gudgeon. Who, besides the spots, is behaving perfectly normally.
Whatever became of yours?