Where did my carbonate hardness go?
And why doesn't my ph crash? I just tested my water parameters for the first time in a long time, and I was surprised to see my carbonate hardness (kh) was less than 10 ppm. Out of the tap it's about 30-40 ppm. My last water change was three and a half weeks ago. The other thing I don't get is that my Ph is 6.9, even though I have a fully stocked tank and basically no buffering capacity whatsoever. In fact, my kh levels have always been in the 30 ppm range, and the Ph never dips below 6.8 or so.
Here is some info about my tank:
- 32 gallon low tech with 40 watts of light
- home to about 20 pencilfish, five sailfin tetras, three farlowellas and a keyhole cichlid
- very, very heavily planted with crypts (it's a jungle - I can barely see the fish)
- no CO2, no water column fertilization, no peat, no carbon: some substrate fertilization with jobes sticks and seachem's root tabs
- water change of maybe 40% every three to four weeks
- hagen test kit that is admittedly a few months past its "best by" date
This may all be a test kit malfunction, but over the years I've been keeping this tank the kh has always been low, and the ph never, ever crashes. Every source I read says I'm supposed to increase the carbonate hardness to guard against ph crashes, but I've never done so, and the ph always stays steady at just below 7.0.
As for the drop in kh, and assuming the test kit is right, could the kh have been taken up by filter bacteria? By plants? I've never gotten a reading that low before, but I did increase the bioload higher than it has ever been by adding sailfin tetras. My working theory is that the increased bioload caused an increase in filter bacteria, which took up the carbonate hardness and caused a decline in kh, but that still doesn't explain the relatively high ph levels.
Fish and plants are happy, so this is a question out of pure curiosity. Does anyone have any thoughts about what might be going on here?