I'm also curious as to what a regular would say...
is there something wrong with me commenting in your thread? even if it doesnt provide you with a solution you need?
I usually feel the more discussion the better! Always happy to have new members and I hope you feel welcome JustFrozen. I think one of the best ways to learn is to participate in other people's threads.
When fishless cycling, "percentage" water changes have no meaning. Aquarists use "partial" water changes as a means of lowering mineral or temperature shock to fish. Our bacteria have no such sensitivity, at least not such that we need to worry about it. So I always say to change as much as you can, right down to the gravel, although in practice I like to "cheat" and leave my cannister filter running, so I don't go below the intake grid. But anytime you bother to do a water change during fishless, its better to do absolutely as much as you can. In most cases you are raising the pH, raising the KH, putting more calcium and iron in there and clearing out both nitrites and nitrates, neither of which help a fishless cycle. The trade-off is that a water change will sometimes cause a "pause" in the processing the bacteria are doing, usually not more than a day or two and often you don't see any pause at all.
The reason you want nitrates out of there is because when NO3 is in solution, a small but fixed percentage of it will be in the form of nitric acid, which is very acidic and is the thing that causes pH to potentially drop whenever nitrification is occurring at a high rate. Depending on the buffering (the KH level) the acid will be more or less able to "move" your pH.
rgrrmg, your problem is that you are not putting enough baking soda in. During fishless cycling, the water is meant to be your "bacterial growing soup" and there is little or no concern about what it will be like later with fish. So there's no harm in dumping in the baking soda, as the excess sodium (which is one of the negatives of it) will be dumped out with the big water change prior to switching your whole system over and getting fish. So I'd dose the baking soda at a rate of an entire tablespoon (3 teaspoons) per 50L of tank volume. That way, your KH should get right on up there and you can then use it as a "leading indicator"... when it works its way down to KH=4 or below then you know you're in danger of the pH crashing and you need to dose it up again.
~~waterdrop~~
/dataguru.org/misc/aquarium/calKH.asp
