Question Acne looking lump on neon tetra's chin/mouth


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Exactly one week ago today, I purchased 7 neon tetras from a LFS.
After a couple hours of acclimating them (floated the bag in tank), I netted them out (to avoid any fish store water from pouring into the tank) and released them into a 10 gallon tank.
Didn't get to see much of them the first they because they were in hiding.
Second day, I noticed one tetra with an acne looking lump on his chin. At first, I thought he had a piece of sand stuck to his chin because it looked almost perfectly round was about the size of a sand grain.
I was worried so I continued to carefully watch them for the past week.
Exactly 7 days later, I now have 3 neon tetras with lumps on their mouths.
The size of the lumps haven't changed, and their behavior doesn't seem abnormal.
They're actually starting to look really comfortable in the tank, less skittish and sky than before.

My tank is 10 gallons with fine sand substrate.
I have two sponge filters running, one on each side the tank.
There was no heater in it until earlier today but the temperature has been reading consistently at 76~80 degrees.
This tank was only up and running for about 1 week when they were put in (so 2 weeks now), but I'm using a sponge filter from my other more established tank (though not by much since it's only 2 months old).
I used API quick start and stress coat before putting the fish in, and have been supplementing with more (1/2 dose every other day).
I've been testing the water daily and the readings are always good (7.2 pH, 0 Ammonia, 0 Nitrite, 0 Nitrate)
They are the only (and first) fish to live in the tank, the only other occupant being one nerite snail (purchased on same day at the same store, but they were in different tanks).
I've vacuumed the sand once this week, and haven't done any water changes yet.
I didn't feed them for the first three days. Now I give them TetraColor tropical flakes, small pinchful twice a day.

It's puzzling because they look and act completely fine, but the lump seems to be contagious.
I also looked up cotton mouth and NTD, but nothing exactly matched what I was seeing.
Am i just not realizing it's NTD because it's at an early stage? They don't have any other spots/growths on their bodies/fins.
Would whatever they have be harmful to a Betta? I was going to add my Betta in after 2 weeks, but I won't if this can affect the betta.
How do I treat this? Or should I just take them back to the store before they all are growing lumps/dying?

Any help/advice would be greatly appreciated!
Thanks in advance :)


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