Alkalinity question

My 75 gallon has been up about a year and a half. Up until about a month ago, I was checking alk, calcium, mag with a red sea test kit. Always had a hard time with the color change on the alk test but I always came up with around 8.5 dkh. I finally got a hanna checker and it shows my alk to be around 6.5 dkh. Last Sunday I mixed up 20 gallons of water for my water change. I decided to check the alk of that water and it was 9.3 dkh. I waited a few hours after the water change and tested the water. I got 6.44 dkh, calcium was 440 and mag was 1360. My questions are: How can I raise alk safely and why would it be that low anyway? I don't dose anything. My corals seem to grow very slowly but also seem healthy. Could low alk be a cause? I have some lps and softies. Been trying to figure this out on my own but I think it's time to get advice from the good people of TRT! Thanks