Best way to kill your fish?

[Black Diamond]

Here's a mystery...

I checked the water and all pass the test with flying colors. The ammonia is bright yellow - and I thought that's what was killing them. But I read the chart wrong, bright yellow is PERFECT and a .00 PPM of ammonia.

WTF is killing then then? Ammonia, nitrate, nitrite and PH are all doing great. Not sure WTF to do now.

Sure it's not ick?