Yes, I can explain(warning, gets a bit techy
). Since you are using baking soda to buffer and slow your pH drop some, it'll be easy to get the hang of why the KH test is helpful.
First, as I believe Dave mentioned, baking soda is adding bicarbs, not Ca carbonates, so there's a difference: Assuming you have some reasonable amount of GH to your tap water (which you can now measure with the GH part of your kit) its still going to be a good thing when you water change, in that it will bring in some Ca and the chemolithoautotrophs (the litho part of the name literally means "eaters of rock" and the Ca is part of that rock they eat!) use that for building structures inside their cells, for use in their cell walls controlling movement of things across the membrane and also its a substance that gets into the biofilm structures built by the bacteria I believe and can help them with their niche in competing with other bacteria etc. So measure your tap GH.
The baking soda (Sodium Bicarbonate) though is "buffering" your soft water. The bicarb part (the HCO3-minus) can bond with the H+ ions in there which are what's measured by a pH kit. As the fishless cycling process puts more and more acids out there, there's more and more H+ and pH heads downward. But it doesn't do it in a linear fashion. Instead, depending on the amount of buffering in your water (which the KH can tell you something about) your H+ ions will be balanced for a period of time until the buffer runs out and then pH will take its more rapid turn downward.
When you only measure pH, you are blind to knowing when this sharper drop is going to happen. When you measure KH (KH is a measure of carbonate hardness or "temporary" hardness, but we can explain that some other time) then you can "see it coming."
In my opinion, the breakpoint to focus on is at about a KH of 4 or so. When you are at 4 and above, then you know you've probably got some days of "buffer" (pun intended) whereas as you pass downward through KH of 3, then 2 and lower, you know the chance for a pH crash is very close, depending on how hard the buildup of nitrites and nitrates is pushing in the acid direction.
Doing the KH test can be a little weird if you're new to it. You fill the tank water to the right level in the test tube. Then you want get ready, near a good light and be ready to watch before starting. Now I'm speaking from the TetraTest KH kit and I believe the API KH one and most others work the same way, but you want to watch that first drop carefully and if it goes yellow on you then you've got ZERO KH, otherwise if its a blue tint then you count "ONE" and go to the next drop. If its a blue tint you count "TWO" and keep going. The drop you're on when it turns yellow is your KH number (in German degrees of KH which is the scale we like to share when talking here rather than the actual bigger ppm number.) Please check your instructions to confirm this as I'm just doing it from memory.
So KH is this wonderful, simple little test, very easy and quick, that works in conjunction with your pH test to give you a much better feel for how your buffering is doing and when to take action prior to a pH crash. Because its easy, you can do it at lots of different points, as a beginner to it, to get a feel for what the adding of baking soda is doing to your bacterial growing water and to watch its buffering effect fade.
Don't forget that the whole reason you're bothering to monitor and alter pH in a fishless cycling tank is because the beneficial bacteria grow fastest at pH 8.0 through 8.4 and in fact their rate of growth varies quite a bit if plotted along a graph of different pH environments. In fishless cycling you are both trying to move your water more towards that optimal growth pH range and also keep it away from the 6.2 stall or 6.0 stop point and of course out of the 5.5 and 9.0 extremes.
Sorry, I kind of "got into it" there, but you can ask questions if you want and I'll try to check back.
~~waterdrop~~
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