Sorry, I realize you probably knew that bit about Ca and Alk being coupled but I just sort of had to include those comments so that the rest of what I was saying would make sense. In general I would say that 8 is indeed better than 7 but I don't think 7 is too low in and of itself. I wouldn't want it any lower, so it's better to shoot for 8 if you're going to to track it.
Most likely this is indeed where your salt is mixing. There may be a slight draw already but its not huge.
I wouldn't say you need to be "concerned" down the road but ultimately keeping track of Ca and Alk is something you do want to put a little attention on. Finding your own comfort zone is key. For me what works is 2-part dosing (I dose Ca in the morning, Alk at night) and then testing once per week to see if I'm adding back at roughly the same rate they are being used up. As corals grow (and clams, and so on), or as you add corals, the rate Ca and Alk are consumed goes up, and thus you have to add more as time goes on. But you don't have to follow this example. Some people just dose weekly. Some people don't dose at all, but my advice is that Ca and Alk are things you do want to dose and test for on a regular basis since relying on your salt mix to make up for it isn't really the best practice - it just can't follow the dynamics of consumption and growth as well as you can.
