I've tested Salifert and Fast Test readings against my colorimeter and the Salifert was almost dead on with all I tested. Fast Test was pretty good with Ammonia; Phosphate gives a good indication of whether there are some or not, but isn't totally accurate as to the actual reading. I didn't have the reagent for nitrite or nitrate, but copper was WAY off on the Fast Test and close but not dead on with the copper. HTH, Marcye