This sounds like me when I first got into the hobby. I put in a TON of ph buffer because it was supposed to buffer it at 8.3. I took some of my tank water to my LFS the next day because it wasn't raising and found out my dKh was 24...
Before you do any dosing, like others have said, you have to test. I wouldn't waste your time with other alk test kits. Go straight to Hanna and forget about the rest. I also use Hanna for phosphate. Make sure to get the ULR tester. I use red sea for calcium, magnesium, and nitrate (only until Hanna comes out with the high range nitrate).
Do yourself a favor and test every day for the next couple of weeks. You'll start to see a trend that shows you what your tank is doing. I tested my alk yesterday and found it at 7.8, but it was at 8 a few days prior. I didn't change anything because during my testing, I found there were some days it would fluctuate a little but then come back in line with what I strive to keep my level at. I bet it'll be at 7.9-8.0 tonight when I test. If it's lower, then I'll wait another day and test to see if my alk consumption has truly gone up. Only make changes once you're sure your tank needs it.
And don't forget to do your research! Buy something after your research it; don't research it after you buy it. Bulk Reef Supply has a nifty series that will definitely help you learn the basics.
https://www.bulkreefsupply.com/content/category/brs-tv/52-weeks-of-reefing