Columbian blue tetra

[Colu]

I had 12columbain blue tetras in a 50gallon  for 14month in the last 4months they have started dieing one after another start  with no symptoms they are eating ok swimming ok but then just start lose balance and  can't swim and are dead with in a day  all my water conditions are fine ph6.8  nitrite 0   ammonia 0  nitrate 10I have very soft water kH 3 GH 4  temp 25.5 treated with maracyn2 and done 2 rounds of general cure and aquarium salt has had no effect one Keeps dieing every and  4 to 6weeks i have have treated for bacterial and parasites   am out of ideas one of my fish stopped eating to night  and is hiding  in the plants the odd thing is i have 10xray tetra  and 2 bristlenose in same tank and have had no problems with them it's only affecting my Columbian blue tetras 

Edited June 10 by Colu

[FrozenFins]

I don't know whats wrong with your fish. But what i can say is to not treat with ich x that will ony be for the worst. Its probably not ich if you only see one white spot. Fish don't alwyas react well when you treat with meds when they are not sick with the specific disease.

[Colu]

One of my other Columbian tetras has acting funny not eating but this one has black marks on body and one white spot on top of the head are here are   some pictures I was thinking possible ammonia poisoning with the black marks I have tested for ammonia daily for last two weeks and it's constantly at zero 

Edited June 10 by Colu

[Colu]

It is only effecting my Columbian blue tetra  I have not loss any x-ray tetra or bristlenose from same tank which is unusual if it were bacterial or parasitic infection 

Edited April 6 by Colu