No red flags. The reagents used for water testing do expire, and they aren't very accurate to start with, but what they lack in 'absolute' accuracy, they make up in 'relative' accuracy.
What this means is that a 7.3pH measurement might really be 7.1 to 7.6pH (and some kits are worse), but now if the same kit then measures 7.0pH on the same day on different water, that difference (0.3pH) is probably quite real and reasonably accurate. Also the 7.0 would be twice as acidic as the 7.3 (logarithmic scale) so this is not a trivial measurement error, especially if the readings are well within the reagent's measurement range. A pH of 7.0 is ten times more acidic than 8.0, so that's a significant relative difference for a test kit to make (unlikely to happen), however note their measurement ranges. Very often, 8.0 falls in the high range test kit and the 7.0 requires a low range test kit, or you might be at the measurement extremes of a wide range test kit. If you're at the extremes, their accuracy drops off. If you're using a Low range and a High range test kit to make the two measurements, then you lose a part of their 'relative' accuracy (because of the different reagent solutions). Then there's some inaccuracy with the light source & direction. The best results are with a medium-strong source of natural daylight in the room, and have a woman look directly down inside the test tube (women have better color accuity than men). I'm sorry for all the detail on the topic, but your problem requires us to mine down into some of the more complex details. Basically, if the readings are sound and the CO2 levels constant, the tank has a kH sink and a pH source (which is usually attributed to a kH source). You approach sounds fine. Use known water, measure, add potential modifiers (substrate etc), and re-measure. Maybe that 'small-size gravel' is aragonite, dolomite or some other leeching mineral (and the acidic water added is crossing an accelerating threshold?). That the buffer you're using is not very stable or that your kH is being chewed up is not too surprising, but that your pH then increases is the interesting puzzle. Perhaps your destination tank has a lot of municipal buffer in it (caustic soda) which boosts the pH? This wouldn't show up in your gH/kH tests, but your water changes should have diluted it by now (perhaps not?). We could use input from others in the group, hopefully from those with chemistry degrees (I only know enough to be dangerous ;~) NetMax On Feb 1, 9:46 pm, nitrocarbon <[email protected]> wrote: > "NetMax" <[email protected]> wrote: > >To be clear, your source water is 7pH, 6dkH, and after putting it into > >the tank at various concentrations, you measure the tank at 8.4pH, > >2-3kH. > > Yes, you understood correctly. That is exactly the situation with this > tank. I just can't explain it. Even this evening the pH had climbed up to > 7.8 until I did a partial water change for 3 hours; now back down to 7.4. > That 7.8 pH is the same level it was when I was using my near-toxic tap water > to begin with! Granted that aside from pH and KH I'm having no other > apparent water troubles with the new water source. > > More background on the tank: It has a mixture of planting substrate, > laterite, and small-size gravel. I run a lot of oxygen through the water > constantly and there is lots of surface agitation. For its size it has > medium plant load, medium bioload, exposure to sunlight during the mornings, > high, but not extremely high light, and a lot of driftwood about 5 months in > the tank (yet I've seen tanks with much more wood). There is no rock other > than two or three small pieces of slate the size of large coins. There is > nothing in the filters but sponges of various porosities, biomedia (Rena > stars and matrix), and a very fine Rena water polishing media pad. There's a > mid size sponge filter, 2 heaters, and no CO2 injection. I haven't been > dosing any ferts for the plants in this tank. I've been holding off on that > until today actually, because I was waiting on the purchase of a CO2 system > for this tank first. There's no hood on the tank, but I'm building one. > That's pretty muc > h it. > > Do any of those things raise red flags? > > I'm afraid I might have to buy a second mater test kit to validate the > results of the first, and run tests in isolated tanks for gravel and the > driftwood. > > ~nc
