2 months - 15 fish dead! What am I doing wrong?

[itiwhetu]

I am still going to say the biggest problem with this tank is pH

 

[StevenF]

The water filter looks a little small what is the size of the tank and what filter are you suing? Is the water surface perfectly flat or do you see ripple or indications water movement at the surface? Or is there any film on the surface of the water.

If there is no or little water movement little oxygen in the air will get in the water and little to know CO2 will get out of the water. Having water movement at the surface is critical to insure stable oxygen and CO2 levels.