Lots of us have nitrates in our water supply. If you test the tap water you may well find that it has significant amounts of nitrates in it. You said your nitrates were high but did not say how high. What is the reading you are getting with your liquid testing kit?
The tank water that you added will be a percentage of the total water so should show that percentage of the tank's reading in the new water unless there is nitrate coming in with the tap water.
An immature bacterial colony will not result in any nitrates at all, it will make it impossible for the ammonia to be processed through to nitrates. Nitrates are only removed from our impounded water systems by water changes. We do not have a full cycle functioning with anaerobic bacteria in a freshwater setup.
The plant should be using up some of the nitrates in your water and gradually reduce the reading unless nitrates are coming in from ammonia conversion or a similar source.

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Fishless cycling