no probable cause mass fish die-out

[DFT]

recently my 55 gal fish tank has been reduced from a population of
18 to a population of only 8.

nitrate is 20
nitrite is 0
hardness is 150
alkalinity is 180
PH is 8.4

there are no signs of disease

one of the tiger barb is injured

i have dosed with antibiotic

 

[libertybelle]

Based on your numbers I’m going to guess you used a test strip for those? Test strips can be fine for checking quickly that everything is going well but when trying to diagnose a problem you really need liquid test kits and, critically, a number for ammonia.

What’s in the tank? What were the symptoms?

If you give us the info from this thread it will help us get to the problem much quicker.
http://www.aquariumadvice.com/forums/f17/before-posting-about-unhealthy-fish-read-this-32451.html