moving my ph up

[joefish]

My tank has been up and running for only 4 months.I'm also new to the reef hoby.I've been following some of the threads on ph,alk,and calcium relationships.I've got the basics that they should go up evenly ,but my tanks alk is at 3.5 meq/l,the calcium is at aprox 400 ppm and the ph at its highest during the day is8.15 to a low at night of7.9.
The person at the LFS said that the tank probably had low magnesium .So I checked and it was low.So I added some magnesium and the ph dropped a little, the magnesium went up, the calcium went up to 440 ppm.
I did a 20% water change and all the numbers went right back to the starting point.
Now the question is how do I get the numbers equal to where
they should be,Or even figure out where the imbalance is,Or is my tank to new to have a correct balance?
I should add that I'm useing a calcium reactor . The ph from the reactor is 6.5.
Any help with this would be greatly apretiated.

[Randy Holmes-Farley]

and all the numbers went right back to the starting point.

That's probably because the starting salt mix doesn't have as much calcium as you want in the tank.

FWIW, I would not have diagnosed the problem as being related to magnesium. The calcium and alkalinity values that you reported (400 ppm and 3.5 meq/L) are perfectly fine. There's no reason to raise them.

The pH is a tad low, but many people keep tanks there. That is especially true of those using reactors like yourself. It is just an unfortunate fact of life with CaCO3/CO2 reactors: the pH will run on the low side.

Some people use limewater along with the reactor to raise the pH. Others do nothing and seem to be fine.