Help - fishes are dying

Hi Everyone,



I need some advise. All my fish are dying. Not sure why all my fish are dying. I did a water test this morning nitrate, nitrite, Ph, Alkalinity, and ammonia results were great. Am I doing something wrong? Can my rocks and sand have something that is killing my fish. Appreciate your help.



Tank Size: 20g

Light: 42 LEDs

Filter: Marineland Penguin 200

Powerhead: 2 AquaClear 20 with aeration

Heater: 100w

Live rock and sand.

Water Temp: 76-78