Pristella Tetras Unwell

[BoltonN]

Evening all, i'm really at a miss with my pristella (x-ray) tetras. I have 8 in a 120L tank along with some red phantom tetras, otos and panda cory. A while back one of them got black marks on them and began to bloat and then eventually died of what i believed to be dropsy as its scales had started to pine cone and it couldn't hold its position in the water. Fast forward a few months and a couple more fish have got these black marks but made a full recovery. Now again i have 3 fish all come out with pretty severe black markings and lost scales, one also looks pretty bloated. I'll attach pictures below but does anyone have any idea what it could be and how to prevent it in the future and the best method of treating the fish that are ill. I have separated the 3 into a hospital tank for the time being to keep a closer eye on them. Perameters for the main tank are as follows:

Ammonia : 0
Nitrite : 0
Nitrate : 10-20
pH : 6.6-6.8
GH : 5
KH : 2
temp : 24.5

[Martinspuddle]

I'd suggest it's a Protozoan parasite, a anti parasite treatment will help clear it up.

[plankton]

How old are they?
I've noticed that all my tetras have gone dark or even black with old age.
@Martinspuddle could well be right though as they have all got the problem at the same time, however, why haven't the phantoms got it as well?
Have the phantoms been beating them up perhaps, and this is secondary?
The water shouldn't be a problem for them.

[BoltonN]

Sorry i've just come back off holiday yesterday. Another of the tetras in the main tank has developed slight black markings around its top fin. Of the 3 in the hospital tank my grandad continued the anti parasite medication however one passed away. The other two seem to be either staying the same or getting slightly better. I've had them for around a 9 months now i'd say and they were only young when i got them. The phantoms seem the more timid of the bunch in the tank so i doubt it, however there is one considerably larger x-ray that gets quite bullish at freeding times.

[plankton]

Sorry to hear you've lost one.
Have the others coloured back up or not?