I agree; unless you have the means and desire to test for relative levels of your various fertilizer components (about $30-$50 per sample, per compound, plus usually a 2 week turnaround time at a water lab) once every couple weeks, you are shooting yourself in the foot by not changing out the res. As PHJ noted, your plants will use ferts at different rates, and you have no way of knowing WHAT exactly is making up your xxx ppm number... Save the headaches! It's easy!