Planted tanks and dead fish.

[tanker]

If you have other fishes, it is probably fish food. I would not be too worried, did it jump out??

 

[jgulecas]

In a heavily planted tank, the water changes are more for the plants than the fish anyways. The plants will consume all of the nitrates.

 

[Star_Rider]

I always thought plants would consume the simpler form of ammonia/ammonium over nitrate but either way.
the nitrate level will drop