dKh slowley dropping

[eleyan]

I Have a 72G reef with the following parameters:

ammonia, nitrites, titrates: 0
PH: 8.2-8.4
calcium 500
Kh: 6-7

originally, my kh was 9-10 and my calcium was low (300 range). I started with kent a/b solution and I got it to 400-450 calcium and 8 kh. After that, I switched to using kalkwesser in my top off water using a float valve. I average 1g a day of top off water with Kalk. My calcium is still rising, and my kh is slowly dripping (it was around 7 for 3 months, and it just got to 6). I though kalk is supposed to maintain both Kh and calcium at the same time.
I know magnesium is linked to the kh/calc balance, but I don't have a Mg test kit. I'm using oceanic salt, and I was using instant ocean before that. I though both had good levels of Mg. I do a 20-25% water change once a month, Should I supplement Mg separately on top of that?

[Andrew]

Kalkwasser won't necessarily keep dKh up. The Hydroxide in Kalk combines with free hydrogen (to form H2O), decreasing the # of Hydrogen ions (relative to hydroxide ions, OH), bringing the pH up (but not alk). Kalk does not contain carbonates, bicarbonates, and other necessary salts/metals such as Magnesium, Strontium, etc. that help prevent drop in alkalinity.

I would recommend add a carbonate/bicarbonate buffer (not at the same time as Kalk, maybe on an alternate day, hours later, etc.).

I would recommend testing Magnesium every month minium if you continue to use Kalk. Regular water changes may or may not supply enough magnesium/trace elements, depending on what salt you use and the rate of uptake of these elements by your system/animals.

HTH

Andrew

[GaryP]

The hydroxide ion in Kalk will also combine with carbonic acid (CO2) to form carbonate, raising KH. Most of the "acid" in sea water is present as carbonic acid. I'm sure there is a combination of both things happening in your system.

eleyan... It sounds like you just need to supplement the kalk with a little buffer. I use baking soda and washing soda at a 6:1 ratio. That's basically the same thing as what is in the Kent A solution, but with a lot lower price.

Gary

[eleyan]

Gary, How much backing soda/washing soda do you use per volume of tank water? and is this also best dripped into the tank like kalk?

[GaryP]

Eleyan,

The amount depends on how much you need to raise your dKH and the volume of your tank. I would suggest some trial and error. Add a small amount and then re-test. Keep adding in small increments until you reach the desired dKH. I make up a stock "A" solution (.5 lb./1 gal. of RO) that I add to my makeup water to pre-dilute it. DO NOT add the solid buffer to your tank without diluting it.

It sounds like you just need to fine tune your dKH occasionally. When I need to do this I usually just siphon some of my A solution through some air tubing into little vacuum eductor port on a Maxijet 12. This gives good mixing and prevents it from creating an area of high concentration that results in a "blizzard."

HTH,
Gary