okay, here's the scenario. Had a reading of 30 nitrate monday with doc wellfishy, did a 18 gal wc on wednesday (had to let the water cure for a while), just opened the fastest kit (expiresin 2008) and tested on wed. Wellfishy still read 20-30 nitrate, fastest read almost 0 checked ph, ammonia and nitrites with both, and got the same readings (pH8.2,am=0 ni-0). I have a 125 with a 20 gal sump (only 1/2 full) and estimate the water volume at 125 to 130 gal. Lotsa live rock (125-140 lbs) 4to5 inch dsb, 2 denitrators and just started building a fuge. My nitrates SHOULD be around 0 and I am trusting the fastest until I can get another one. tested 2 other tanks with both kits, and got the different reading on nitrates, but everything else the same. I'll go with the fastest for now until I can locate a salifert, or get one shipped.