What is this

[black ghost]

The two females pictured are Red Phantoms. The youngster is H. amandae.

[plankton]

I was going to say that I've never seen juvenile red phantoms, but I should imagine that have some definition of the black mark on the sides, but that definitely looks like an ember.
May, just maybe, there were some eggs on a plant you got which hatched and they've been around for some time, they aren't big fish (and would easily get lost in your tank) so you could've missed them.
Anyway, if they're free, what does it matter?

[Art]

The spark tetra, H. amandae. A little fish for nano.

[Marty.h]

They must have snuck in then as ive never bought any that I know of especially not that small

[Art]

It's possible. With bought plants or other fish.
As a small fry.

https://www.aquarium-lutter.de/funkensa ... n-amandae/