Clown Loach Black Spot

[marcan]

Hi!

I have 3 clown loaches in my main tank and I noticed that one my loach has a black spot on his dorsal fin.
There's only one spot.

3 weeks ago, I bought 3 more clown loaches. They are in a quarantine tank to make sure they are okay before adding them to the main tank. None of the new loaches had spot when i bought them. Today i noticed that one my new loach has a black spot too on his dorsal fin.

Is it normal? Is it because they come from a specific area in the world where clown loach develop a spot in this area?
I didn't buy the loach at the same LFS and they have never been in contact together.

Water stats for both tank:
Ph 7.4
Ammonia 0
Nitrite 0
Nitrate 20
Tanks have been running for 4 months.

Thank you in advance

Here's a picture:

 

[lilfishie]

i am not 100% sure but it looks like there colouring to me

 

[marcan]

Both are about 2 or 3 inches long.
They are small and probably young.

 

[marcan]

Thanks you lilfishie

Anyone thinks it's something else?

 

[The-Wolf]

I too think it is a natural colouration mark