Just regular Arm & Hammer Baking Soda?
Yes. Any baking grade baking soda. I've never seen one with additives, but if you ran into one I'd skip it.
And for the calcium chloride...it sounds like that is a product used for multiple uses, but will often have extra ingredients in its formulation. Do I need to review an MSDS on a product? Or do you have a safe, cheap and readily available suggestion?
I'm not sure the MSDS has the percentages of ingredients, but normally it'll be on the label or web page if purchased online. I don't know if it's a law but all sources I've seen list the ingredients and percentage. In another thread I think a person found some with 2 or 3 other ingredients, like potash I think. Check out each to see if there's any danger and at what level.
Almost all these things produce salt, so you have to have a good idea you're not adding too much salt. For example, baking soda produces salt after reacting. Doing the computations (not me, but a real chemist named Dr Roddy on Koiphen) a level of 10,000 ppm KH with baking soda could result in too high a salt level for fish. 10,000 ppm is crazy high, so really, baking soda is hardly worth worrying about. A deicer could have high salt, but unlikely these days. Different products have different percentages of calcium chloride, which is what you're after. When doing your estimates you take the percentage into account. Like if you found something with 50% calcium chloride the estimate would be to add 2 pounds per 1000 gals. 100% calcium chloride would be 1 pound per 1000 gals. That's ballpark of course.
The GH is less important than the KH. And the KH is less important if pH isn't swinging. For all I know your pond could be stable at 6.9 and really, if it stayed stable and you were willing to test daily, you could stay there and not add KH. Originally, the 6.5 pH was too low for the bacteria so that was an issue. Having the buffer just makes a keepers life simpler. So what I'm saying is if you're wanting to raise KH I would not wait on being ready to raise GH. That can be done later. KH will reduce most of the pH swing, if it is swinging, which I'd bet it is.
Over long periods of time salt can build up in a pond, as well as many other minerals. This is almost never an actual issue with normal water gardens. In fish loads it could be a problem, but they do water changes often for other issues so mineral build up would not be an issue. But on the other hand, someone like me in the desert and 6% humidity I can't just totally forget about water changes. Somewhere down the line you should consider trickle water changes. That's where you have an overflow and use a drip head 24/7 into the pond to do a 10% weekly water change. It evens everything out a bit.
Of course most of this is about improving conditions, not about saving the fishes lives this week. Koi and goldfish can take a lot and most have to. My guess is probably fewer than 10% of pond keepers have ever test their water. Most of them don't really have any idea what the numbers mean. You'll hear things like "my pond water is good and the fish are healthy" when actually the fish are going through hell and in a few years they're pushed over the edge, die and no one can figure out why. Not judging, I had fish for lunch. Humans use animals. Koi and goldfish were specifically created by humans to be used with little consideration for them. But a little effort reduces the chance of having to see pets die every 4 or 5 years. Just wanted to give a little perspective on why anyone would go to this trouble when thousands of people never have any problem with their pond...always perfect...until it isn't of course.
