Go to the grocery store, buy Distilled water etc.
Given that you are doing at leats 50% weekly water changes, adding a mere 35 ppm at most per week, the max build up possible, and this assumes no uptake at all, and no denitrification, and also no fish contribution would be 70ppm.
However, a simple 70-80% water change would resolve that higher level, it's just a 55 gallon tank so that might be another 10-15 gal is all.
Something is causing the tank to either not take up the nutrients etc, or the test kits are wrong. In any event, is there something wrong with the tank otherwise?
Use your eyes and plant health/fish etc to make changes, do not base them on the test kit which you/I/anyone is unsure of.
They can cause as many issues as a high NO3, but generally cause far more issues than high real NO3 levels.
Those are much easier to contend with, you just do a larger water change, focus on why the plants are not taking up more NO3, generally poor CO2, need to clean filters , deep vacuum old sediment etc and perhaps dose a tad less if the tap is high NO3/PO4 etc.
That focus helps the plants, whereas simple reducing NO3?
That does not really help anything much. NO3 is not nearly as toxic as many like to suggest......and as many have never even bothered to test using KNO3..........
If the other parameters, CO2, good plant biomass, health tank etc otherwise are addressed, then adding higher NO3's is not that big of an issue.
I've done it time and time again with some rare sensitive fish for several years.
I think a 20-40ppm range is fine, and huge target most can hit with/without a test kit that's calibrated.
But you can go lower if you chose or slightly higher.
Just generally target 20-30ppm.
PO4 is no cause for any concern, it's not toxic and causes to ill effects on plants or fish or us for that matter. It can be 10 ppm etc, so like K+, unless it's really way way out there, it's not a cause for any concern, just wasteful perhaps.