Tap water should always be de-chlorinated before being added to the tank. The Amquel+ will accomplish that for you. When doing water changes...I usually add Amquel+ to the tap water, mix it up a bit, and then add that water to the tank.
Since your tap water's nitrates are high, then you might want to consider getting some live plants. Something like anacharis is very easy to grow, and will help consume some of the nitrates. Do you have any live plants right now?
Also, as Zagz said, uneaten food will increase nitrates...so making sure to not overfeed will help to slow down the nitrate build-up.
I'm not sure about the daily addition of Amquel+. I guess you're doing that because of the nitrates, but that shouldn't be necessary and will become quite expensive.
Good luck!