Tiger Barbs Dieing One By One

[SnoutUK]

Over the past 4 days, one of my six tiger barbs has died, its always through the night time.
I have checked the water, results are as follows...
Ammonia - 0.0
Nitrite - 0.1
Nitrate - 25
pH - 8.5
Does anyone know why they are dropping like flies? My other fish (Swordtails and Mollies) are absolutely fine. I have checked them all over for signs of disease and they all seem very well, even the tiger barbs before they die!

Please somebody help!

 

[TerraDreamer]

Well, you're showing some nitrite, but the level seems low at 0.1. What test kit are you using...I have the Freshwater Master Test Kit by API and it won't read 0.1. Your pH is pretty high; not sure if barbs are sensitive to that or not. I really don't have an answer, but at least I'm bumping you back toward the top. Hopefully someone can help. I suggest a water change in the mean time to lower your Nitrite.

EDIT: You said your fish are dying one by one, but mentioned only one fish death. If it was only one, it could simply be a matter of him being the weakest...perhaps bullied to death?

 

[Marshall..]

It looks like your ph is very high so gradually reduce this,you could put bogwood in the tank to help reduce the ph gradually.
But with it being one by one it sounds like they have formed a pecking order and one keeps getting it.
I suggest getting that ph down,more water changes so the nitrite wont sneak up and then add as many tiger barbs your aquarium will
hold to make it difficult for them to pick on the weakest.