2 Part Issue

[thegrun]

I've been dosing two part for several years. I upsized my SPS dominated mixed reef tank on 4-24-15 from a 50 gallon true water volume system to a 145 gallon true water volume tank. I kept the same dosing quantities (170 mL per day) in the new tank as I had in the old on the theory that since I moved most of my hard corals from the old system into the new system. I have been slowly having to increase my dosing amounts over the past two months I assumed to compensate for the new growth. The last two weeks I have needed to increase the dosing to 184 mL of two part, and even with that My alkalinity has been dropping daily about 1.8-2.0 dKh with the Calcium dropping only about 5-10ppm. There is a little bit of sand clumping (but not bad) but no other signs of overdosing like calcium precipitation on the glass.
Do you think I am overdosing the two part and thus contributing to the sudden increase in alkalinity demand or do you think the coral growth could account for the change. There is more hard coral growth going on now but the sudden daily drops in alkalinity from 8.5 dKh to 6.8 has me wondering. Sorry for the long post, but I tried to give as much background as possible.
-Mike

[bertoni]

Did you add any live rock or expose any more surface for coralline to grow? Coralline can consume 2-3 dKH per day, in my experience. Also, the substrate might be serving as nucleation sites for precipitation. That should drop over time, though, and it's already been a couple of months. How much coral is in the system? I agree that more growth is a very likely cause.

[thegrun]

I am seeing quite a lot of new coralline on the new rock and back wall, I had no idea it could consume that much alkalinity. The ratio of alkalinity to calcium consumption seemed a little high, but that could just be testing margin of error.

[bertoni]

I suspect that the coralline accounts for the difference, but you could watch for signs of abiotic precipitation. That can be an issue, too.