Yes. These will determine which kind of hardness you already have and which you may be lacking and which is disappearing as time rolls on. You should test brand new tap water for both, as well as aged aquarium water.
Usually the answer ends up being crushed coral gravel added to the filtration, but I can't say that will be your case until we get your hardness sorted out.
In my case, the tap water comes out with gases dissolved in it. Then, as it degases, the pH crashes from 7.3-ish down to around 6.5-ish. Horrid roller coaster and I fixed it, here in washington where there is virtually no testable hardness of any kind, with crushed coral gravel in my filters. This keeps my pH stable at around 7.2, but I can't do massive water changes because the coral doesn't dissolve as fast as the water degases and I get a double yo-yo from all the new soft water going hard later. It's crazy, but I hope that an easy fix is on the way for you.
