"Koi-Lo" none wrote in :

Someone suggested the gravel was unclean. I can't see how he can have
nitrites with nothing alive in the tank...?!?!?!? The store clerk

You don't need living things in your aquarium in order to cycle it. Any
decaying organic matter or waste will kick start the cycle. You do need a
long-term waste producer to sustain it well over time, but not to get it
started.

Old used gravel or unwashed ornaments will contain trace amounts of dead
organic material which will produce ammonia as it decays.

Bagged store bought live plant sand substrates generally come with a good
quantity of dead organic material including dead bacteria, which will
produce ammonia as it decays.

Some people add pure ammonia directly to their tank to get the cycle going.

Cat **** will cycle an aquarium--not as uncommon as you might think,
especially in aquariums that have been sitting around dry for a long while.
You could cycle it with your own urine if you're vile enough to really
consider it.

Throwing fish food into an empty aquarium will cycle it too, but you'll
have to wait for sufficient digester bacteria to accumulate before their is
significant food for nitrifying bacteria.

Probably none of these circumstances are what the original poster has
experienced, but my point is there are a lot of reasons why nitrites may be
present despite their being no fish in the tank. So long as the levels are
not very high, there is really no reason to worry about where they came
from.