Just like the title says, my nitrates have disappeared and I have one dead fish. I set the tank up a little over two months ago, and cycled it by seeding with rock from a smaller, well established tank. A month ago, the water tested at 8.2 ph, 0 ammonia and nitrites, and ~ 20 ppm nitrates. Tonight, I noticed my other fish, 1 auratus, bumble bee and yellow lab, and 2 acei, all juveniles and surprisingly, no serious bullying from the auratus. eating my other yellow lab. they all seem to be doing well and seemhealthy but now one is gone. I have no idea why it died. It showed no signs yesterday that it wasn't doing well.

Anyway, I decided to check my water again and everything is perfect, except I have no nitrates. Thinking I must have tested wrong, I repeated the entire test again, with the same results. The only thing different over the last few days is I haven't run my lights but only for a few hours tota be because I noticed some algae starting to grow. Could the lack of lighting be the culprit? Will my nitrates bounce back without cycling again? I sure hope so but something tells me no.

Any advice would be appreciated.

Do you have coral in the tank or is it just fish? Some fish will get sick and die or if your tank is too small, they will die because of stress from not having enough space. Some foods may have gone bad or not been the right kind for that fish and it starved. Lighting wouldnt be a culprit unless the oxygen depletion from coral (if you have any) slowing the rate of oxygen production down.
So questions: How big is your tank? Whats the numbers for the parameters? Do you have crabs and snails to help finish off the fish food you have in there? Do you have any macro algae? How often and what kind of foods do you feed?