What exactly are your nitrate levels? 20ppm in the tap is a tad high. What you -could- do is try replacing some of the water in the tank with reverse osmosis or bottled water. they have zero nitrates.
In my 80 gallon my nitrates are well over 200ppm. well, as far as I can tell on the Aquarium Pharm test kit anyway.
I am in the process of doing a series of consecutive water changes - yesterday I did 20 gallons, waited a half an hour and doing another 20 gallons. I am going to do the same thing again today. Gotta love the python.