Kh Low and dropping

Tested everything in my tank very carefully:

Ph 8.3
Calcium 400 ppm
Alk 89 KH
Magnesium 1280 ppm
Nitrites 0 ppm
Nitrates < 1 ppm
Salinity 1.020
Temp 78-79


Alk of my salt mix (Oceanic), is 120-125 ppm KH.

Why then did my alk drop after a 20-25% water change and why is it low to begin with?

 

First your salinity is way too low 1.025-1.026 for a reef tank. Are you measuring DKH? What test kits are you using? I remember oceanic salt being low in buffers? That was 2 years ago I think. You can try a 2 part Alk/Cal supplement. I do this along with kalk in my top off water and it keeps my 46 gallon with a DKH of 9 and Cal-420-440. I have a heavy stocked tank


Does you PH swing much from morning to night? Low ALk will cause this

 

Last time I tested Oceanic it wasn't too low, but I went in dkH, and it was like 8, so it is right on par if you ask me.

You are using RO water I take it?
What test kit are you using?
It shouldn't have dropped unless somehow you had a CaCO3 precipitation occur but you would have noticed that...

 

Why is your salinity so low. It should be 1.025 or 1.026. Did you try adding an alkalinity suppliment?

All these measurements will be on the decline constantly. Riase your magnesium to 1400 and the next day or two raise your alkalinity.

 

Who said it is a reef tank? .020 is fine for fish only.

 

Using API test kit. I'm just getting into corals. Yes, I'll try raising SG and see what that does. thx

 

Raising the SG should not affect the Alkalinity for the simple fact that the 20-25% water change will dilute into the existing water. You may get a nominal effect but nothing substantial. With where your calc and mag are you should be able to maintain alk at 10-12 dkh. I would use Reef builder to raise it and then monitor for the fall off rate.

 

How are you getting your mag readings?
Even though the numbers you PUT up are good, I would still suspect your mag to be off. if you can keep a figure of 9 you should be fine.