Weird dark spots and dark scale outlines

[Othnus]

Hi everyone, 

Maybe you could help me out. So I took after a second hand aquarium and made it my own. It had very few fish left in it when I got it. So it had two of these tetras. One had like this big black blob in the middle of it's head, that looks like a third eye. Both of them started to develop these small black/darkish spots, which over time started to grow bigger and bigger. Now I've noticed, that my green neon tetras and panda corries have this mild darkish scale outlines, they don't have these black spots on them, but the outlines are noticible. The coloring looks very similar, just not as intense.

The bottom one tetra had absotule no spots on them.

I'm following ~5 day water change in two different ways. First I change ~30% of water and the second time I do ~50% with gravel cleaning/sucking out the trash.

pH is 7.4

KH and GH is about 14 degrees, but all fish are locally bread and used to this.

NO2 and NO3 is 0

I feed them flakes and frozen food, which I rotate too in order for my pets to get varied nutrition.

Tank is 240L. It get's ~10h of lamp lights, since it's heavyly planted.

Any ideas guys?

 

Thanks in advance!

Edited December 25, 2020 by Othnus

[Colu]

Can you get better picture thanks

[Othnus]

14 hours ago, Colu said:

Can you get better picture thanks

Let me grab my camera.

[Othnus]

19 hours ago, Colu said:

Can you get better picture thanks

Here's some of better quality pictures.

Edited December 26, 2020 by Othnus
Typo

[Colu]

It could be a parasitic infection  with secondary bacterial infection treat with API general cure and maracyn

Edited December 26, 2020 by Colu