KH, GH, TH ?! Which one to test for?

My plants are still struggling after 2 months. I have 55 watts over 29 gal for 9 hrs a day, use Flourish Excel, plant sticks, and started liquid potassium a few days ago (could that have caused a recent rise (0.4) in pH?). My ammonia and nitrite are always zero, with nitrate around 5.0. The only thing that I haven't looked at is hardness. This part seems very confusing. I read about calcium and magnesium hardness, carbonate hardness, total hardness and whatever. Which one do I test for?. Hopefully I will only need to buy one more test kit. Thanks for the help. Tackful