OP, when you thought your tank had cycled, ammonia and nitrite was 0ppm, but what was your nitrate level at that point?
I used Seachem stability when cycling my tank (~300 including sump) and it took much longer than I anticpated. It wasn't so much the product, but my bio load was so low it took a VERY long time to get traces on ammonia. Finally, I got traces of ammonia never greater than .5ppm, then traces of nitrite at no more than .5ppm. My nitrate never exceeds 5ppm, and I noticed slightly less (with API kit) since I did a recent large water change. My point here is that I had low nitrate to begin with that was produced based on my low bio load. Something to keep in mind, hence I was asking about what nitrate reading you had when cycled.
BTW, using Prime should not alter your readings. I don't know if that's true for equivalent products. Good luck.