Yeah, 5g is about right. I'm having a similar issue with my 125g, the alkalinty is kinda low but my mag is fine and my calcium is off the charts.
I've never used marin so I don't know if it's the salt or that your corals are absorbing too many nutrients too fast for you to replenish.
Wish i could be of more help. :-(
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.