As I am still seeing levels, low as they are, of ammonia and nitrite, I have not been testing anything else. PH has not changed from 8.3 but I think that is because of my rock and sand. I have not been checking calcium yet as I have nothing in this tank, barring the rock, that will use it. In the reading I have done I see that alkalinity until stable will fluctuate so I have not been checking it either untill I see no ammonia. I could be wrong. It hasn't happened yet this year though
As far as the plenum.. is that not an anearobic area under your sand bed that has minimal water flow to lower PH in that area to increase the breakdown of nitrogen? My 10 gal nano has never had good calcium levels... until I started using a nano filter on the change water. Not as good as an RO but still removes 60-70% of dissolved solids. Running two nano's in series and I am finally able to get calcium above 350. I'll let you know where the big tank stands when I get it checked.