Hereditary fin rot?

[LoachFINatic]

I have 12 bleeding heart tetras in my 50 gallon aquarium.

I bought them in 2 batches. One was a batch of 6 large bleeding hearts at the start of this year, and the other 6 were purchased a couple of months ago at a smaller size.

I have never ever had any issues with my first batch nipping at all. They have been peaceful, healthy and gorgeous. Then, I got this second batch and they soon got what would appear to be fin rot.

I never witness any nipping what so ever, and these are not the smallest in the group, they were healthy looking and great! One of them has it worse than the other, but they are still looking healthy besides this.

None of the big ones have ever had this, and it is only the small ones that have ever shown any signs of it.

What is a slightly strange "coincidence" is that my friend has some bleeding hearts from exactly the same batch, has never experienced much nipping and yet hers have exactly the same, she has even lost some of hers.

My water is clean and params are good, as is hers, so that doesn't really explain it, and I think it would be a bit of a strange coincidence?

Could it be hereditary, or is it just a really odd coincidence?

Thanks!