I agree that the difference between 40 and 80 is quite alot. As long as my parameters aren't fluctuating rapidly and are within "safe" limits I assume everything is healthy
Probably. However, from the info I've gathered, it would seem that 40ppm KH is really close to, or under, the "safe" limit that would keep things stable. 80 ppm KH should be great, though. Right in the middle of the "optimal" range most sources quote.
I'm not sure if comparing tap water to tank water is the simplest way to look at changes in pH and kH
For pH, I'd agree (see below), but for KH, I'm not so sure, based on personal experience, and what I've read.
Me, personally, I've got two tanks, a 10g and a 46g. They've got somewhat different environments, significantly different lighting, and somwhat different kinds of fish loads (type, not just amount). Over the last seven months, the KH of both tanks has remained rock steady, and matches my tap water's KH exactly (as far as I can measure it, that is). Of course, my KH is pretty high (~180ppm).
From the things I've read, this steady KH seems to be a 'normal' condition, if the KH is sufficiently high (over 60ppm or so).
The pH of my tap water straight from the faucet is 8.8+ (highest measurable rating in my kit). The guy at my LFS and a local expert from the Boston Aquarium Society advised me that this is largely because of chlorine and chloramines,
Could be. I've never heard any other things to back this up, but that doesn't mean much. Most of what I've heard about pH changing when tap water 'sits' is related to CO2.
I took the experts word for it that Boston water doesn't have sufficient buffering capacity to sustain water at that pH as chlorine dissipates.
From what I understand, that's probably true, although it doesn't really have much to do with the chlorine, it's just a property of the carbonates' buffering capability.
To the best of my knowledge, the buffers reflected in your KH readings will tend to 'drive' your pH to a certain point (which is the same principal behind many of the pH raising/lowering products -- although those are usually phosphate buffers). These numbers are from memory, but I seem to recall seeing repeatedly that a KH of 3 degrees (~55ppm) will tend to drive pH to around 7.2-7.4, and a KH of 10 degrees (~180ppm) will drive pH to around 7.8-8.0. Therefore, to sustain a pH of 8.8, you'd need ... well, a pretty darned high KH (or a lot of other buffers).
On the other hand, according to the research I've done, sometimes in tap water with low levels of dissolved CO2 tap water can have relatively high pH (not enough CO2 to acidify water) and low kH (not a lot of bicarb available either to buffer the water).
From what I've gathered (mostly from 'plant people'), that is true for pH. That is, all else being equal, adding CO2 to the water will cause the pH to drop. However, at the same time, everything I've seen tends to say that the KH stays constant (assuming, again, that it's not very low to start with). Also, from what I've read, 'usually' tap water will contain more CO2 than your atmosphere. That is, as you let tap water sit, the CO2 concentration goes down, and pH rises.
The exact formula for the relationship, by the way, is
CO2 (in ppm) = 3*KH (in degrees)*10^(7-pH).
(which I got from
this excellent article, and that relationship assumes the absence of other buffers, particularly phosphate buffers).
and for the curious, I think typical 'stabilized tap water' conditions are 3-4 ppm CO2.
Bol-- can fish die from low kH?
Technically, I think it's unlikely, but it's kind of a question of semantics ...
Or was Kiri suggesting that she had a rapid fluctuation in her pH from having a low kH?
That's my understand of how it works. The fish probably don't die directly because of a low KH -- but the low KH will cause a 'pH crash', and the effects of that will harm the fish.
In other words,
if you could sustain a tank with, say, 10ppm KH AND 7.6 pH, your fish probably won't die. However, 10ppm KH makes it quite difficult to naturally maintain a 7.6 pH.
So, if A causes B, and B kills the fish, then is A or B the 'killer'? Technically, B is, but you could make an argument for A.
