Hah! That's how I knew my RO filter was bad; I was mixing nutes with tap water like I always do for the extra calcium, and on a whim I tested the RO as long as I had the TDS meter out anyway. RO came in at 110, and the tap water is consistently around 175. Went out and bought new filters the next day.
See, I thought I remembered changing them pretty recently, but evidently not. This time I got smart; I put a piece of painter's tape on the outside of the unit and wrote the date down.
OP - if you do decide to go with tap instead of RO, I second the suggestion that you get the water analysis. The geology of wherever you live will make a big difference in what minerals you have in your water. For example, if you live in an area where the geology is all limestone, your water will be high in calcium but may have almost no magnesium at all - so you'll still need to supplement your mag, and using calmag may not be your best choice because you're just wasting "PPM headroom" on extra calcium you don't need.