First off, scrap the Hagen's for better test kits
Now that that's out of the way, let's get to the problem solving:
1. How old is your tank, what's your tank's net water volume and parameters? (Ca, Mg, dkH, etc)
2. What is the set up like? Any additional filtration? (UV, Phosban, Ozone, Skimmer, etc)
3. What kind of maintanance do you employ?
4. What is your dosing schedule?
5. Any previous problems with the tank?