when there is bacteria/poop buildup, what goes up? is it the nitrites?

A good overview of the aquarium nitrogen cycle is here:
The Aquarium Nitrogen Cycle

To summarize, you first add nitrogen compounds into your tank primarily as fish food. Whether the food is eaten and then pooped or just rots in the bottom, ammonia will be formed. If you have a type of bacteria called nitrosomonas, then the ammonia will be consumed and converted into nitrites. If you have a type of bacteria called nitrobacter, the nitrites will be consumed and converted to nitrates. Of these three (ammonia, nitrites, and nitrates), the nitrates are by far the least toxic to your fish.

A few additional notes:
-If you have live plants, they will consume the ammonia, nitrites, and nitrates as fertilizer.
-The above mentioned bacteria will eventually colonize your aquarium automatically if you don't do anything to kill them off.
- If you do not have live plants, then there is nothing to consume the nitrates and you will definitely need to do partial water changes to remove them.
- If you do have live plants, they will consume nitrates, but you may still need to do some partial water changes unless you have a lot of plants per fish.

There's lots more, but I hope this answers your question.

-Tim

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