a fish dies every day help

A River Murray Cod is a predator, so it's possible.

To clarify, you said you got all 0s on your ammonia, nitrite, and nitrate tests. Is that correct? If so, something is wrong.
 
And rain water, aka distilled water, might cause issues
 
What is the roof made of? Generally speaking, the rain water used for aquariums should NOT be coming off a roof. Collecting it from direct fall is safe, but coming off a roof, it's going to collect particulates at best, and chemicals that you can't test for at worst.
 
What is the roof made of? Generally speaking, the rain water used for aquariums should NOT be coming off a roof. Collecting it from direct fall is safe, but coming off a roof, it's going to collect particulates at best, and chemicals that you can't test for at worst.

Not only could it collect particulates and chemicals but also animal droppings and urine could get into the tank.

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A River Murray Cod is a predator, so it's possible.

To clarify, you said you got all 0s on your ammonia, nitrite, and nitrate tests. Is that correct? If so, something is wrong.
Normally I agree but the nitrogenous waste from just 11 small fish in over 2,000 gallons of water is not going to register on the test kits available to hobbyists. That is A LOT of dilution power.
 
additionally, if there are no readings for nitrates (being at 0), the tank is uncycled.
Not necessarily. The nitrate may be reading 0 ppm, but they may actually be so low that they are not able to be registered by the API kit which is only able to go as low as 5 ppm. The OP has a lot of water and a very light stocking right now.

Right now I'm most concerned with how the tank water is collected. There's no telling what the water is picking up along the roof. I'd set something up where you're getting direct rainfall and not any runoff.
 
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