Too summarize you can attack this two ways.
Nitrate is the direct by product of waste, weather food, excremental, or respiratory. So you can either try to lower the amount of waste, deep clean, less feeding, more water changes, or attack the nitrates directly. Resin media like Purigen, live plants, algae scrubber.
The Nitrates in your tap water is a problem, but treating your tap water with Prime should detoxify it.
That is the other thing to consider. With a binding agent like Prime, or other solution, the Nitrate could be binded, so it's not toxic, but it will still show up in a test. These binging chemicals usually only last 12-24 hours though.
With your tap water being what it is, you just may have to be a daily water change guy.
Some Purigen in a micron cloth bag. It will fit perfectly in your Aquaclear.
I've been using Purigen in my heavily stocked 55 for a few months, and I am very very happy with it. I not only see a difference in my testing, but in my fish as well.
Sent from my iPhone using Aquarium Advice