Caulastrea sp. (Candy cane) is an easy coral.
I don't know about Nickleback, but I can tell you several reasons why API tests are crap. 1) Short shelf life. They will probably expire while you own them. 2) Very hard to interpret. 3) A real pain the the rear to perform - shake this for 2 minutes, let that sit for whatever... 4) Even if you jump through all the hoops the right way, they're dreadfully inaccurate.
They are probably the most widely sold, because they're the CHEAPEST. People will spend thousands on fragile livestock, but balk at $10 or $20 more for a good set of test kits. That's like buying a $100,000 sports car and being too cheap to change the oil regularly.
Red Sea, Salifert, Seachem - they're all good. Pick a brand and buy them before you continue. Spending a few dollars on a good set of testing kits and a refractometer will save you thousands in mistakes later. (Don't forget the calibration fluid for the refractometer, I haven't seen one accurately factory calibrated in some years.) I'm partial to Seachem test kits because they come with a reference solution so you can check for yourself that the kit is accurate AND your method is correct. However Red Sea and Salifert are good choices too - just check the expiration dates when you buy. Seachem kits have a pretty long shelf life and have lot numbers so if you are ever in doubt, you can call Seachem (a Georgia company, BTW) and they can check the lot numbers and if you experience a problem they will replace the reagents.
What is National Geographic Sand? I have never heard of it and I googled it and no joy. Is it aragonite? If it isn't, then don't use it. Spend a few dollars and get the right stuff and be done with it. Otherwise you'll be spinning your wheels even more.
If that Caulastrea is failing, there must be a reason. You're best to find out why and address it, before 'trying another one'.
Jenn