How do I keep PH stable?

[DraggingTail]

It looks like nobody answered your question about how much to add. While true you have nothing alive to worry about it, if you want the reassurance that you can create a predictable salt water for future water changes before you have something to kill I think this is reasonable. When I mix salt water, and it is off, I go to BRS site, use their calculators. Type in information (remember to use all gallons, not just what is in the tank), and it will tell you. Bicarb for increase alk without ph changes, ash for alk plus increase ph. Kalk for ato and maintence (no calculator for this, dial it in with trial and error, no need until something alive is using it).

I have a Duncan, Acro and Acan now

 

[tiggs]

Personally, I never chase ph unless it drops below acceptable levels. This did happen occasionally in my situation, since I run a skimmerless nano without a reverse lighting cycle refugium that's located in a room that's window doesn't open a ton. My solution was to setup an air pump with limewood air stone in a back chamber and have it run on a reverse cycle from my lights. It's not quite the micro-bubbling method that's gaining popularity in the nano world, since the bubbles never actually enter my display tank, but it helps greatly reduce the swings. This changed my ph swing from 8.2 --> 7.6 to 8.2 --> 7.95

One thing to be cautious of when using additives to regulate ph is that it typically also makes your alk fluctuate. In some cases, that's perfectly fine. In others, it may be the difference between harmony and chaos if not monitored.