Fish dying in 135g planted tank

[FinsandFeathers]

My wife started a 135g planted tank several weeks ago. Tank was cycled properly before adding fish.
Substrate is Black Diamond blasting sand.
Filter is Fluval FX6 canister.
Parameters are: Ammonia, 0 ppm; Nitrate, 0 ppm; Nitrate 20 ppm; PH 7.2; GH & KH both 75-100.
In the course of about 3 weeks, we've lost probably 50 small fish; neon tetras, guppies, kuhlis, swordtails, dwarf puffers, cories . . .
No visible signs of disease on any fish.
Does anyone have any ideas what the problem may be?

 

[coralbandit]

How did you acclimate the fish ?
Did they all come from same place ?
Did you add all of them at once?
Can you run carbon in your filter?
I might be inclined to run carbon for a week or so ?
Are the deaths random or are they time related to when the fish were added [ like within 2 weeks of being added ] ?