Hello there MACHO... just curious about a few things.
1. Do you have a FTS? - seeing is believing in most cases.
2. Do you have a sandbed? If so, have you ever siphoned it while performing your water changes? (I believe this technique helped me lower my phosphates and nitrates)
3. How old is your aquarium?
4. How long were you using the carbon dosing method? (What lead to this method?)
Most chemical changes in an aquarium with living organic material - should take some time to change.
IMO, if you broke your 70 gallon water changes into 10 gallon daily water changes for one week...then a 20 gallon water change every week for instance. The issue should balance out. Depending on your readings, try smaller water changes every 3 days of so.
However, that's taking into the account of a system without biopellets and carbon dosing.
**** If I'm not mistaken most people that carbon dose or use biopellet do so without periodic water changes .
5. Which brand of salt are you mixing? Some salts have similar results to carbon dosing.... Aquaforest has the Probiotic Reef Salt which could help. Tropic Marin has Bio Actif salt....
Welp...that's my TWO PENNIES. Good luck.
- Larry