Im having a confusing battle with my parameters; so thought I would describe the situation and see if any thoughts on what is going on. I'll list the tank details at the bottom of the post. I returned from the TG holiday after being away 4 days and found that my Ca had suddenly dropped to about 275. Not even measurable on the Red Sea test. It had been locked in at 425 for at least the past few months (test weekly and add ABC powder daily). In the weeks that have followed I have continued and stepped up the dosing but I am still measuring at that same low. I can't seem move the needle. Meanwhile Mg and KH are steady at good levels. Well, there were two dramatic changes to the system that happened at about the same time of that infamous weekend when this all came about; One was that while I was away my topoff sensor failed. The water level ran down so low that I came home to find the display tank full of microbubbles from air the return pump was taking in. I must have got home literally just in time; although perhaps not really because I also lost a fish which I haven't found yet. Im not sure if those two events are linked but its the first fish I've lost in this system and they all looked normal/healthy when I left (kept telling me to go have fun and don't even worry about a thing). Well that same day in order to make myself feel better I decided to start up the new pico tank as I was only waiting on the sand and it arrived while I was away. And I had too much left over sand to discard so I decided to add sand to the frag tank in my main system. Now I am thinking that its the adding live sand that might have thrown my Ca for a loop, but I don't read any online material to suggest this cause and effect. I'd really like to understand that especially because I was planning on also adding more live sand to a fuge area in the sump but not if that is overly risky.