Please help - Cardinal tetras dying

I have a 15 gallon fluval flex that had been running a month with 4 cardinal tetras and 4 rosy barbs.
Just under two weeks ago I gave the rosy barbs to a friend and bought another 9 tetras and decided to have an all cardinal terra species tank and so I had only gone from 8 fish to 13.
Yesterday I found that one of the fishes bodies had become very pale on the red part and looked like it was going to die.
I had a bit of hope and I left it until the morning and when I woke up it had died.
The rest of the fish looked fine, so whilst saddened I took the dead fish out and then made sure my other fish were okay.
Everything has been okay until today when I have just found two dead fish and another two in the similar state of the one that died, very pale on the red section and struggling to swim as usual. Parameters are all good however ph is slightly higher than the recomened range by 0.6.
I’m worried that this is going to happen to all of my fish now
Is there anything I can do?
All help is appreciated
Thank you very much
Jack

 

May be neon tetra disease which can cause discoloration and if you have noticed anything, spine deformities as well. This is spread when fish eat parts of a fish that has died with it. If you notice any separate immediately it cannot be helped

 

I'd get the infected ones out.

 

How to Manage Deadly Neon Tetra Disease
Fish with NTD often have these symptoms:
Discoloration in a certain part of the body
Difficulty swimming
Staying away from the school
There is no cure if your fish is confirmed to have it the most humane way is to euthanize it. If left in the tank to die and a fish so much as nibbles on it before you can remove it it’s possible to have spread

 

Update, just seen a parasite/white thing that had a long white body and tiny legs, about 1-2mm big on the filter intake, I’ve done a google and it looks like the photo I have attached or very similar. Found another dead fish, getting very sad now. Can only think that the fish I got from the shop last had the parasite/disease and have gave it to over fish.
I haven’t got another tank other fish can go in.
I guess there done for, very sad and if there is anything I can do please tell me.
Thank you

 

Update, just seen a parasite/white thing that had a long white body and tiny legs, about 1-2mm big on the filter intake, I’ve done a google and it looks like the photo I have attached or very similar. Found another dead fish, getting very sad now. Can only think that the fish I got from the shop last had the parasite/disease and have gave it to over fish.
I haven’t got another tank other fish can go in.
I guess there done for, very sad and if there is anything I can do please tell me.
Thank you

Since you don’t have another tank if possible try to make a bet of some kind that can hang in the tank so that if a sick fish passes if cannot come in contact with others. These a small quarantine net at Petsmart for $6

 

Update, just seen a parasite/white thing that had a long white body and tiny legs, about 1-2mm big on the filter intake, I’ve done a google and it looks like the photo I have attached or very similar. Found another dead fish, getting very sad now. Can only think that the fish I got from the shop last had the parasite/disease and have gave it to over fish.
I haven’t got another tank other fish can go in.
I guess there done for, very sad and if there is anything I can do please tell me.
Thank you

Whatever you saw is probably not the culprit, might just be bug larvae of some kind probably not a threat
Edit: It’s a dragonfly/damselfly larvae they will eat fish their own size so be cautious

 

Did you acclimate properly? How did you acclimate?

I don't think it is NTD, most of the time neons are affected by that said, NTD can affect other fish if that fish ate the dead body of a diseased fish.

 

Fish with infections like these should be removed right away.

I doubt it is NTD (that only got its (mis)name cause it was ID-ed on a neon) but it simply is severe and not good.

As stated in another thread I think the healthy ones only have a chance if the ill ones are removed and euthanised.

Treating is useless in this stage