My guess would be that the bio wheel on your filter has enough greenery
growing on it to eat the nitrates produced in your tank. Also a good
amount could be growing in the tank and getting eaten by the pl*co.
Something is eating it - I seriously doubt that a year's worth of
nitrate buildup is just sitting in your tank with no bad effects on
anything.

Be careful with your tank! It's nice to have a balanced setup and not
have to work to keep fish...don't mess it up by disturbing the balance
too much! Good job - and good luck.

-Daniel