I had always used Salifert test kits and loved them until the DKh test kit screw up back in June of 2007. Then the manufacturer just disappeared and closed his thread on RC instead of addressing the issue, so I have lost all faith in Saliferts DKh test. I have been using API for several months now and have validated my results against Elos, and it appears to be spot on. I also don't like the 1 degree increments instead of the .1 but have figured out that if you get a minor change and add one more drop and get a full change, you are probably right in between the two full reading so you can guesstimate a .5 difference. It is accurate enough for the cheap price tag associated with APT tests. As other has said though their PO4, Nitrate, and even Ca tests are very sketchy at best. My API Ca test constantly reads 560 in my system, but when tested with Elos or Salifert tests it reads 410-430. NO3 reads 60 in my tank, Elos and Salifert read <10. I think the best PO4 test without getting a Hanna or Hach is the Deltec test. Unfortunately this thing costs $80.00, but you can get just the refills when you need more solutions instead of buying a whole new kit, so that will bring the cost down over time.
