Phantom Tetras dying

[fraoch]

Hi everyone,

Having a real issue with my phantom tetras with 3 dead in the past 4 days. The only symptoms they exhibit are faster breathing, a loss of colour and not eating/hiding in the corners and within 24 hours they die.
I had a good look at the most recently dead one with no obvious signs of illness, even cut open the stomach area to see if there were any worms, looked behind the gills and in the mouth for parasites- nothing!

Tank parameters:
28 gallon planted
Temp 24C/76F
Ammonia-0
Nitrite-0
Nitrate-20 (usually lower at 10 but I am doing a big WC today)
pH- 6.8
Hardness-0 (Should I use a tonic to increase hardness a bit?)

There are 6 remaining phantoms, none of which are showing symptoms right now and all other fish are healthy which are:
8 celestial danios
1 BN pleco
Pair of macmasters apistos
3 false jullii cory
2 chain loaches

I added a half dose of salt to the tank yesterday when the 3rd tetra started showing symptoms and I will be doing a 50% water change today

Any ideas at all whats causing this?

 

[fraoch]

Also forgot to say that they have just been treated with Esha 2000 last week because the 2 male tetras had a scrap and cut one of them so I dosed as a precaution but there were no signs of ill fish at that point and I've done a 40% WC since then and added carbon filtration back in