[ruki]

Originally Posted by guaiac_boy

First of all, in a planted tank, who cares about nitrifying bacteria? The plants take up so much that most of us have to add it back in.

With billions of such bacteria, I think this is another blow the assumption that C02/carbonic acid/pH is an isolated, independent system behind the pH change dissolved CO2 chart. Secondary changes will be going on at least in the background because a large biosystem has been throttled back by a pH change. We could really use an inexpensive way to directly measure the dissolved C02 to know what is going on.

I think Edward's approach of keeping fish in a KH of zero is interesting, and reveals that a lot of what we pass around as "fact" probably isn't.

Humans recognise patterns, whether valid or not. Perhaps I could become rich starting up an aquatic plant religion where people started noticing good things happen when they send me donations to construct a temple to a yet-to-be-defined diety.

An understanding of the principles behind the physiology and chemistry is more important than trying to get parameter "X" to a certain value.

But that's not how most people "learn". It's easier to have a religious faith based upon pH crash killing fish, when it could be something else (i.e. pH was actually lower, but it was an effect not the cause of the fish deaths.) They saw dead fish, lower pH, therefore confirmation!

Regarding the KH/CO2/pH chart...... It works perfectly well as long as carbonic acid from CO2 is the only acid in the system and carbonates are the only buffers. Since this is never true, the chart can never be expected to be accurate.

This is conceptally nasty because hardnes and the resulting equilibrium states between a buffer and the dissolved ion/acids of the mineral plus the interaction between free H and free OH and is conceptually quite difficult.

Decomposition of fish food, fish waste, plant matter, driftwood, and other compounds will inevitably introduce organic acids that accumulate with time.

And other stuff in the aquarium which dissolves much more readily when pH drops. You can and probably do have significantly more dissolved "crud" in the water if you drop the pH from 6.5 to 5.5. That could also be a factor in the 'pH crash' fish deaths.

I suspect that most of what gets reported around the internet as a "pH crash" is actually an ammonia spike. A low plant mass with an overstocked tank and a poorly functioning biofilter is a perfect setup. Nothing kills fish more quickly. A sudden increase in CO2 is also pretty impressive. I've seen this kill fish in a matter of minutes.

That might be. By the time one gets to measuring this with test kits, the ammonia spike may have gone away since the system can handle a certain base rate which will absorb temporay spikes. People notice the next day they have dead fish, but their ammonia and nitrates test out at zero.

Being vertebrates, the biochemistry that takes place in a fish is pretty similar to our own. Hemoglobin is the carrier of oxygen in the blood stream and it looses its ability to carry oxygen when CO2 levels get too high.

Because hemoglobin can't dump CO2 into the air when the air is already nearly saturated with CO2. The same thing can happen to fish. In this way, too much CO2 can directly kill fish via a fairly simple to understand mechanism.

When humans experience a gradual increase in CO2 (such as in lung disease), blood pH goes down and our kidneys compensate by producing more bicarbonate and dumping more acid.

Don't think the kidneys can produce bicarbonate. Other functions in the body will dissolve bone if neccesary to keep pH from getting too low.

I think Edward is right that high CO2 levels can be more toxic than low pH levels. For most people though, it's hard to understand the difference. This is probalby due to the fact that we're all accustomed to equating pH to CO2.

Because most people don't have a science background, so they stuck memorizing religious type rules such delta pH change equals dissolved CO2. The biggest problem is that people don't understand that this is an estimator with a +/- error range since this is never included in the charts to make them easier to understand. How can one calibrate this? Without such calibration, we don't understand the margin of approximation (i.e. error) in what we read off the chart.

Still, there is a certain pH below which the fish can't adapt. This exact point differs between species.

Even this is a nasty experiment. In your tank you could have something that dissolves in the water only at a very low pH and that is what really killed the fish, even though you can only measure that the fish started dieing off at pH 5.3. You would need a bare tank with just fish and multiple acids in different tanks to see if a common low pH killed the fish.[/quote]

Whew, what a novel....... sorry

One needs a novel to resolve each sub question!