High Nitrates are generally a sign of a biologically "dirty" tank, even though yours looks well mantained. The fact that you have not found the dead fish tells me you had decayed fish carcases & most likely had an ammonia spike.
Tommy
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Eheim Pimp #204.
58 gal planted Oceanic
Red Turquiose Discus, Japonica shrimp, assorted Blue Eyed Rainbows, Tetras, Hatchets, etc.
10 gal low tech planted Cherry shrimp tank
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