Need help stabilizing!

[Mr.Rocc]

So I have a 4-5 month old tank filled with 10 frags( 3 across, 1 monti, 3 zoa, 1 acan, 1 hammer, 1 chalice), and 1 clown fish. Its a 40 gallon IM. All of my basic tests are spot on. pH is 8.4, ammonia 0, nitrite 0, phosphate 0, nitrates 5. However the alk calc and mg I cannot stabilize. My alk is at 8.4dkh from 9.4dkh last Saturday. My mg is at 1240ppm from 1350ppm last Saturday. However calc went from 450ppm - 440ppm.
Now there is no real visualize differences and my tank looks great. I also double tested to compare numbers and they were consistent. Can anyone help me come up with a way to stabilize this tank.

 

[brandon429]

http://reef2reef.com/threads/the-pr...done-in-reefing-is-from-test-misreads.253391/

consider that, double testing of your own kits to verify has issues. the clues here are no impacts within the tank you are reacting too like loss of growth or animals, but making changes only based on unverified test readings. You may have a problem but we wanted to show cross verification needed before action.

Just something to consider! just having corals/frags in the tank isn't the calcium and alk demand, its when they (and coralline) are growing and using it, and we can see the additions and new growths that really state a usage issue